Distribution of glycine, gamma-aminobutyric acid, glutamate decarboxylase, and gamma-aminobutyric acid transaminase in rabbit and mudpuppy retinas

Amacrine cell-mediated input to bipolar cells: Variations on a common mechanistic theme

Feedback is a ubiquitous feature of neural circuits in the mammalian central nervous system (CNS). Analogous to pure electronic circuits, neuronal feedback provides either a positive or negative influence on the output of upstream components/neurons. Although the particulars (i.e., connectivity, physiological encoding/processing/signaling) of circuits in higher areas of the brain are often unclear, the inner retina proves an excellent model for studying both the anatomy and physiology of feedback circuits within the functional context of visual processing. Inner retinal feedback to bipolar cells is almost entirely mediated by a single class of interneurons, the amacrine cells. Although this might sound like a simple circuit arrangement with an equally simple function, anatomical, molecular, and functional evidence suggest that amacrine cells represent an extremely diverse class of CNS interneurons that contribute to a variety of retinal processes. In this review, I classify the amacrine cells according to their anatomical output synapses and target cell(s) (i.e., bipolar cells, ganglion cells, and/or amacrine cells) and discuss specifically our current understandings of amacrine cell-mediated feedback and output to bipolar cells on the synaptic, cellular, and circuit levels, while drawing connections to visual processing.

Changes of amino acid concentrations in the rat vestibular nuclei after inferior cerebellar peduncle transection

Although there is a close relationship between the vestibular nuclear complex (VNC) and the cerebellum, little is known about the contribution of cerebellar inputs to amino acid neurotransmission in the VNC. Microdissection of freeze-dried brain sections and high-performance liquid chromatography (HPLC) were combined to measure changes of amino acid concentrations within the VNC of rats following transection of the cerebellovestibular connections in the inferior cerebellar peduncle. Distributions of 12 amino acids within the VNC at 2, 4, 7, and 30 days after surgery were compared with those for control and sham-lesioned rats. Concentrations of gamma-aminobutyric acid (GABA) decreased by 2 days after unilateral peduncle transection in nearly all VNC regions on the lesioned side and to lesser extents on the unlesioned side and showed partial recovery up to 30 days postsurgery. Asymmetries between the two sides of the VNC were maintained through 30 days. Glutamate concentrations were reduced bilaterally in virtually all regions of the VNC by 2 days and showed complete recovery in most VNC regions by 30 days. Glutamine concentrations increased, starting 2 days after surgery, especially on the lesioned side, so that there was asymmetry generally opposite that of glutamate. Concentrations of taurine, aspartate, and glycine also underwent partially reversible changes after peduncle transection. The results suggest that GABA and glutamate are prominent neurotransmitters in bilateral projections from the cerebellum to the VNC and that amino acid metabolism in the VNC is strongly influenced by its cerebellar connections.

GABA and GABA-transporter (GAT-1) immunoreactivities in the retina of the salmon (Salmo salar L.)

Putative GABAergic elements in the retina of the Atlantic salmon have been identified by immunohistochemistry, utilising polyclonal antisera against gamma-aminobutyric acid (GABA) and the GABA transporter GAT-1. Cell types immunoreactive (ir) for GABA comprise horizontal cells, amacrine cells, displaced amacrine cells in the ganglion cell layer, displaced amacrine cells in the inner plexiform layer (interstitial cells), and Müller cells. In addition, a GABA-immunonegative type of interstitial cell was also identified. In the inner plexiform layer, GABAir fibres were organised in sublayers that were strikingly similar to the sublayering of GAT-1ir fibres. GAT-1ir cell bodies comprise amacrine cells and displaced amacrine cells that may represent a subpopulation of the GABAir ones. In view of the very similar sublayering of GABAir and GAT-ir fibres in the IPL we suggest that a similar type of GABA transporter, that can be recognised with antibodies against rat GAT-1, is present at least in the dendrites of all GABAir amacrine cells but is not expressed in the cell bodies of all GABAir cells.

Pharmacology of the GABAB receptor in amphibian retina

Amacrine and ganglion cells in the amphibian retina contain GABAB, as well as GABAA, receptors. Baclofen, a GABAB agonist, hyperpolarizes the dark membrane potential of these third order neurons and makes their light responses more transient. GABAB receptors in the retina have a similar agonist profile to GABAB receptors described at other sites in the brain. Namely, preferential activation by the R-enantiomer of baclofen, and agonist sensitivity in the order 3-aminopropylphosphinic acid > baclofen >> 3-aminopropylphosphonic acid. The GABAB receptor was not activated by 4-aminobutylphosphonic acid. Several antagonists, such as phaclofen, saclofen, and 2-hydroxysaclofen, were ineffective in the amphibian retina. However, CGP35348 blocked the action of applied baclofen and produced effects on the light response that were opposite to those of baclofen. Applied agonists and antagonists support the hypothesis that GABAB receptors serve to regulate the balance of sustained and transient signals to the inner retina.

Aspartate aminotransferase and glutaminase activities in rat olfactory bulb and cochlear nucleus; comparisons with retina and with concentrations of substrate and product amino acids

The quantitative distributions of aspartate aminotransferase and glutaminase were mapped in subregions of olfactory bulb and cochlear nucleus of rat, and were compared with similar data for retina and with the distributions of their substrate and product amino acids aspartate, glutamate, and glutamine. The distributions of both enzymes paralleled that of aspartate in the olfactory bulb and that of glutamate in the cochlear nucleus. In retina (excluding inner segments), there were similarities between aspartate aminotransferase and both glutamate and aspartate distributions. The distribution of gamma-aminobutyrate (GABA) was similar to those of both enzymes in olfactory bulb, to aspartate aminotransferase in cochlear nucleus, and to glutaminase in retina (excluding inner segments). The results are consistent with significant involvement of aspartate aminotransferase, especially the cytosolic isoenzyme, and glutaminase in accumulation of the neurotransmitter amino acids glutamate, aspartate, and GABA, although with preferential accumulation of different amino acids in different brain regions.

gamma-Aminobutyric acid system in developing and degenerating mouse retina

Freeze-dried sections (14 microns thick) of retinal layers were prepared from mice with retinal degeneration (C3H strain) and control mice (C57BL strain). The weighed sections (2-30 ng dry weight) were analyzed using our microassay methods. In the control retina, gamma-aminobutyric acid (GABA) concentration and glutamate decarboxylase (GAD) activity, on a dry weight basis, increased from birth to 9 weeks of age and decreased slightly at 20 weeks. In the degenerated retina, the levels of GABA and GAD activity were higher at birth than in the control retina, and continued to increase until 20 weeks of age, at which time the GAD activity reached a markedly high level. This increase was found when the total GABA and GAD levels per retina were determined. In the normal retinal layers, GABA and GAD were confined primarily to the inner plexiform layer. In the degenerated retina, GAD activity gradually increased in the inner layers during postnatal development, but by 20 weeks the increase was most prominent in the inner part of inner nuclear layer and in the outer part of inner plexiform layer. GABA transaminase activity and its distribution were not much different in both normal and degenerated retinas during development.

Quantitative distribution of six amino acids in rat retinal layers

Concentrations of glutamate, aspartate, glutamine, glycine, GABA, and taurine were determined in samples microdissected from rat retinal layers and assayed by HPLC. Glutamate and glutamine were relatively high in the inner nuclear (INL) and ganglion cell (GCL) layers; aspartate was relatively high in the outer nuclear layer (ONL), outer plexiform layer, and INL. Distributions of glutamate and aspartate did not correlate well with those of enzymes involved in their metabolism. Glycine and GABA were highest in the inner plexiform layer, with increasing concentrations through the INL, and were relatively high in the GCL. Taurine was highest in the ONL.

GABA concentration and GAD activity levels in normal and degenerated retinas from mice

Freeze-dried sections (14 microns thick) were prepared from mice with normal (C57BL strain) and degenerated (C3H strain) retinas. GABA concentration and GAD activity were determined in the microsamples (1.8-20 ng dry weight) of retinal layers and sublayers, using an enzymatic amplication reaction, NADP cycling. GABA was distributed over all layers of normal retina with a broad concentration peak covering both inner nuclear and plexiform layers. In contrast, GAD activity was mostly localized in the inner plexiform layer. GABA concentration was similar in one-fourth of the sublayers of each inner nuclear or plexiform layer. GAD activity was highest in the innermost sublayer of the inner nuclear layer. An increasing gradient of GAD activity was present in the inward direction in the inner plexiform layer. In the degenerated retina, lacking in photoreceptors, the inner nuclear and plexiform layers remained, and GABA and GAD levels in these layers were similar to those in normal retina.

Distributions of the activities of aspartate aminotransferase isoenzymes in rat retinal layers

Distributions of the activities of the cytosolic (cAAT) and mitochondrial (mAAT) isoenzymes of aspartate aminotransferase were determined in rat retinal layers. cAAT was highest in the photoreceptor inner segments, inner nuclear layer and inner plexiform layer; mAAT was highest in the inner segments. The high activity in the inner segments indicates that both isoenzymes are involved in energy metabolism in addition to a possible role in neurotransmission.

Distribution of three enzymes of gamma-aminobutyric acid metabolism in monkey retina

The distributions of glutamate decarboxylase (EC 4.1.1.15), gamma-aminobutyric acid transaminase (EC 2.6.1.19), and succinate semialdehyde dehydrogenase (EC 1.2.1.24) were determined in monkey retina. The decarboxylase was almost restricted to the inner plexiform layer. The transaminase was also highest in this layer, but activities were 40% as high in the adjacent third of the inner nuclear layer and in the ganglion cell and fiber layers. Succinate semialdehyde dehydrogenase was distributed very differently. Although it also showed a peak of activity in the inner plexiform layer, there was a second equal peak in the photoreceptor inner segment layer and a smaller peak in the outer plexiform layer, regions where both gamma-aminobutyric acid transaminase and glutamate decarboxylase were essentially absent.