Neurotransmitters retinal

A quantitatively important pathway of cysteine catabolism in animals is oxidation to cysteine sulfinate (Fig. 24-25, reaction z),450 a two-step hydroxyl-ation requiring 02, NADPH or NADH, and Fe2+. Cysteine sulfinic acid can be further oxidized to cyste-ic acid (cysteine sulfonate),454 which can be decarbox-ylated to taurine. The latter is a component of bile salts (Fig. 22-16) and is one of the most abundant free amino acids in human tissues 455-457 Its concentration is high in excitable tissues, and it may be a neurotransmitter (Chapter 30). Taurine may have a special function in retinal photoreceptor cells. It is an essential dietary amino acid for cats, who may die of heart failure in its absence,458 and under some conditions for humans.459 In many marine invertebrates, teleosts, and amphibians taurine serves as a regulator of osmotic pressure, its concentration decreasing in fresh water and increasing in salt water. A similar role has been suggested for taurine in mammalian hearts. A chronically low concentration of Na+ leads to increased taurine.460 Taurine can be reduced to isethionic acid... 

HVA Cav 1.4 L-type CtlF retinal neurons, immune cells tonic neurotransmitter release, T lymphocyte activation DHPs blind... 

Below the inner segment is the soma and nucleus, which connects at its base to the axon and synaptic terminal. Photoreceptors release glutamate at ribbon synapses (Heidelberger et al., 2005). Synaptic ribbons are specialized for sustained release of neurotransmitter and are also found in the terminals of retinal bipolar cells, as well as vestibular and cochlear hair cells. Synaptic ribbons receive their name because of their planar strnctnre in photoreceptor terminals, although bipolar and hair cell ribbons are more spherical in shape. 

Muller cells are major sites for the uptake and removal of neurotransmitters, most notably glutamate and GABA. Glutamate transport into neurons is smaller and slower than transport into Muller cells and thus uptake into Muller cells is the principal mechanism responsible for the initial removal of extracellular glutamate following synaptic activation (Pow, 2001). In addition to neurotransmitter transporters, Muller cells possess neurotransmitter receptors and can release neuroactive substances (e.g., ATP) (Newman, 2004). Thus, activity of retinal neurons can influence Muller cells and Muller cell activity can in turn influence adjacent neurons. 

Glutamate is the major excitatory retinal neurotransmitter in retina. It is released by photoreceptors, bipolar cells, and ganglion cells (Sharma and Ehinger, 2003). Normally, the released glutamate remains in the synaptic cleft only for a short time (a few milliseconds). If glutamate levels remain elevated for a prolonged period of time, this can excite neurons to death. This mechanism of cell death is referred to as excitotoxicity. 

Fig. 7. Schematic drawing of a transverse section through the forebrain depicting pathways likely to use glutamate as a neurotransmitter. I = principal subcortical afferents to the thalamus from. somatosensory relay nuclei and the spinal cord (a), cerebellar nuclei (h). and retina (c) 2 = intrinsic neurons and retinal inputs to the hypothalamus 3 = thalamocortical inputs 4 = corticothalamic inputs 5 = cortical inputs to the basal ganglia and other areas in the brainstem and spinal cord 6 = associational and commi.ssural connections in the cerebral cortex. For further details, see Sections 3.5-3.9.

Glutamate s role as a neurotransmitter in the vertebrate retina is reviewed by Barnstable (1993), Brandstatter et al. (1998) and Lo et al. (1998). As the cell bodies of different retinal cell types are in different laminae (Fig. 10), we can assign which general cell types express which glutamate receptor subunits. However, there are different subsets of the same cell class, e.g., there are at least 10 different types of on- and off-bipolars, and multiple subtypes of the other cell classes (Stevens, 1998). Without cell-type markers and double-labelling studies, ISH can not differentiate these. The cones and rods release glutamate onto the bipolar cells only off-bipolars use ionotropic receptors at this synapse on-bipolars use the metabotropic receptor mGluR6 instead. The distribution of NMDA and non-NMDA receptor mRNAs in the retina is summarized in Fig. 10. 

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