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Biomedical subjects

D M Lam

Publications and source records attributed to D M Lam.

108 records · Page 6Linked to original sources
PubMed1978

GABA-ergic pathways in the goldfish retina.

A high-affinity uptake mechanism for [3H]-gamma-aminobutyric acid (GABA) has been localized to type H1 cone horizontal cells and type Ab pyriform amacrine cells in the retina of the goldfish by light and electron microscopy autoradiography. By stimulating isolated retinas with colored lights during incubation we have been able to use [3H]-GABA uptake as a probe of light-evoked changes in membrane potential. All colors of lights increase and darkness decreases [3H]-GABA uptake by H1 cone horizontal cells. Our model of voltage dependence of GABA uptake predicts that all colors of light should hyperpolarize H1 cone horizontal cells and other investigators have shown by intracellular recording and dye-marking that type H1 cone horizontal cells hyperpolarize to all wavelengths of light. We have also obtained evidence that dark-induced depolarization of cone horizontal cells leads to release of GABA. Type Ab pyriform amacrine cells show maximal [3H]-GABA uptake in darkness and when exposed to green or blue lights, but red lights dramatically suppress uptake. We predict these neurons to be red-depolarizing, and recent intracellular recordings and dye-marking by Famiglietti et al. ('77) support our conclusions. Synaptic relations of apparently GABA-ergic neurons were investigated in the electron microscope. We propose type H1 cone horizontal cells to be both pre- and post-synaptic to red-sensitive cones and type Ab pyriform amacrine cells to be both pre- and post-synaptic to red-sensitive center-depolarizing bipolar cells.

Animals↗
PubMed1978

gamma-Aminobutyric acid: a neurotransmitter candidate for cone horizontal cells of the catfish retina.

In the catfish retina, horizontal cells that receive inputs exclusively from red-sensitive cones are the only neurons that accumulate exogenous gamma-aminobutyric acid under our experimental conditions. When isolated eyecups are perfused with bicuculline methochloride, an antagonist of postsynaptic gamma-aminobutyric acid receptors, responses of cone photoreceptors to a field of light (3 mm in diameter) become much slower and approach those to a small spot of light (0.3 mm). In addition, bicuculline methochloride decreases the frequency responses of cone horizontal cells to a field of light. These findings indicate that, in the catfish retina, feedback synapses from cone horizontal cells to cones are chemically mediated and may use gamma-aminobutyric acid as a neurotransmitter. Our results also confirm the hypothesis that, in the catfish retina, a function of the negative feedback is to improve the frequency responses of the system.

Action Potentials↗
PubMed1978

Biochemical studies of isolated glial (Müller) cells from the turtle retina.

A method has been developed for the preparation of large numbers of glial (Muller) cells from the turtle retina. After proteolytic dissociation of the retina, Muller cells were separated from retinal neurons by velocity sedimentation at unit gravity. Fractions containing >90 percent morphologically identifiable Muller cells were prepared by this procedure. Fractions containing only Muller cells were obtained by drawing selected cells individually into a micropipette under visual observation. Biochemical analyses of isolated Muller cells showed that (a) these cells did not synthesize and accumulate acetylcholine, gamma-aminobutyric acid, or catecholamines when incubated with appropriate radioactive precursors; (b) the specific activities of choline acetyltransferase (EC 2.3.1.6), glutamate decarboxylase (EC 4.1.1.15), and tyrosine hydroxylase (EC 1.14.16.2) in these cells were less than 2 percent of those found in the retina; (c) Muller cells, however, contained high activities of transmitter degrading enzymes-acetylcholinesterase (EC 3.1.1.7) and gamma-aminobutyrate- transamine (EC 2.6.1.19); and (d) the cells also possessed high levels of two presumably glial-specific-enzymes-glutamine synthetase (EC 6.3.1.2) and carbonic anhydrase (EC 4.2.1.1). These results, together with other findings, suggest that Muller cells are not capable of neurotransmitter syntheses but possess the enzymes necessary for two important roles in the retina: (a) the inactivation of certain transmitters after synaptic transmission by uptake and degradation, and (b) the maintenance of acid-base balance and the provision of a stable microenvironment in the retina by the removal of metabolic products such as carbon dioxide and ammonia.

Acetylcholine↗
PubMed1972

Biosynthesis of acetylcholine in turtle photoreceptors.

For determination of possible neurotransmitters synthesized by photoreceptor cells, turtle retinas were dissociated into single cells with proteolytic enzymes. These cells were partially separated by velocity sedimentation to yield a fraction rich in photoreceptors. Individual photoreceptor cells were then sucked into a micropipette and incubated with labeled precursors of known or suspected neurotransmitters. After incubation, the radioactive products were analyzed by high-voltage electrophoresis. Of all the chemicals tested, turtle photoreceptor cells synthesized only acetylcholine, suggesting that these cells may be cholinergic.

Acetylcholine↗
PubMed1972

The biosynthesis and content of gamma-aminobutyric acid in the goldifsh retina.

Goldfish retinas incubated with L-glutamate-(14)C (UL) were found to synthesize gamma-aminobutyric acid-(14)C (GABA-(14)C) The accumulation of newly synthesized GABA was enhanced by physiological stimulation of the retina with flashing light; and this increase was directly proportional to the logarithm of the light intensity. The total GABA content was also higher in light-stimulated than in dark-adapted retinas, although the glutamate content remained unchanged No differences were found in the cell-free activities of glutamate decarboxylase (EC 4 1.1 15) and GABA-glutamate transaminase (EC 2.6.1.19) extracted from light-stimulated and dark-adapted retinas. These findings, together with other physiological and morphologcal evidence, suggest that GABA plays a functional role in synaptic transmission in the goldfish retina

Amino Acids↗
PubMed1971

The uptake of ( - 3 H) aminobutyric acid in the goldfish retina.

After goldfish retinas had been incubated for 1 hr with [gamma-(3)H]aminobutyric acid, we found by autoradiography that the label was localized to a few restricted types of retinal cells. In particular, external and internal horizontal cells from light-stimulated retinas were more heavily labeled than corresponding cells from retinas kept in darkness. Some other cells and tissues in the retina also incorporated the labeled acid. Light stimulation, however, did not cause a pronounced change in the amount of label associated with these cells. Among these were some heavily labeled cells on the vitreal side of the inner nuclear layer, and scattered grains associated with the ganglion cell and optic nerve layers. Electrophoresis of retinal extracts after incubation with the labeled acid also showed that light-stimulated retinas contained about 40-100% more radioactivity than retinas kept in darkness, and that 90% of this activity remained as [gamma-(3)H]aminobutyric acid. The role of the acid in the retina is not known; it is not clear if horizontal cells normally synthesize or store it. The stimulation-dependent accumulation of the labeled acid into horizontal cells suggests that it plays a functional role in these cells.

Aminobutyrates↗
PubMed1970

The separation, physical characterization, and differentiation kinetics of spermatogonial cells of the mouse.

By means of the velocity sedimentation technique for cell separation, single cell suspensions from the testes of the mouse could be separated into at least seven peaks each with a different sedimentation velocity. These were named and characterized as follows: alpha (12 mm/hr); beta (10 mm/hr); gamma (6.5 mm/hr); delta (4.3 mm/hr); theta (broad peak with median 2.5 mm/hr); kappa (1.1 mm/hr); lambda (0.75 mm/hr). Tritiated thymidine was injected into thirty groups of mice. The spermatogonial cells of each group were separated at one hour and then at daily intervals, and the acid insoluble activity of each fraction was measured. This method enabled us to determine the differentiation patterns of mouse spermatogenesis by following the thymidine label with time. It was found that the spermatogonia and primary spermatocytes formed the theta peak with S-phase spermatogonia and primary spermatocytes having a similar sedimentation velocity of 2.1 mm/hr. The beta cells were identified as late pachytenes and diplotene cells, the gamma cells as secondary spermatocytes, the delta cells as earliest spermatids, the kappa cells as spermatids, and the lambda cells as mature spermatozoa.

Animals↗
PubMedDate not supplied

Interactions between enkephalin and GABA in avian retina.

In addition to conventional neurotransmitters such as acetylcholine, dopamine, glycine and gamma-aminobutyric acid (GABA), a number of peptide-immunoreactive substances have recently been localized in the vertebrate retina. The functional roles of these retinal peptides and their interactions with conventional neurotransmitters are largely unknown. We have previously shown that exogenous opiates affect both the release of GABA and the firing patterns of ganglion cells in the goldfish retina, and we have now begun a systematic characterization of the opioid pathways in the chicken retina, because, among vertebrate retinas, avian retinas contain the highest concentration of enkephalins. Monoclonal antibodies specific for enkephalin have been used to demonstrate that a subpopulation of enkephalin-containing amacrine cells exists in the chicken retina. This retina also synthesizes Met-enkephalin and releases it on cell depolarization. The enkephalin-induced inhibition of GABA release in goldfish retina led us to examine whether similar interactions occur in chicken, and if so, whether enkephalins and GABA coexist in the same amacrine cells. Our results, presented here, indicate that exogenous enkephalins do indeed inhibit GABA release in the chicken retina. Surprisingly, we found that although some amacrine cells contain both enkephalin and GABA, others contain only one or the other.

Animals↗