Characterization of serine's inhibitory action on neurons in the mudpuppy retina

Monitoring neurotransmitter release from isolated retinas using online microdialysis-capillary electrophoresis

Release of neurotransmitters and other primary amine-containing analytes from intact, isolated larval salamander (Ambystoma tigrinum) retinas maintained in a 6.5-microL perfusion chamber was monitored using online microdialysis-capillary electrophoresis with laser-induced fluorescence detection (CE-LIF). Primary amines were derivatized online with o-phthaldialdehyde (OPA) and beta-mercaptoethanol. With the use of overlapping injections, the perfusate was sampled every approximately 10 s. Although separation conditions were optimized using 20 mM hydroxypropyl-gamma-cyclodextrin (HP-gamma-CD) for a number of important neuromessengers including D- and L-serine, D- and L-asparate, glutamate, GABA, serotonin, dopamine, norepinephrine, and taurine, only glutamate (0.48 +/- 0.27 microM), GABA (0.25 +/- 0.12 microM), taurine (5.5 +/- 2.1 microM), and l-serine (2.8 +/- 1.0 microM) were identified in the perfusate. Elevated levels of glutamate, GABA, and taurine were detected during stimulation with 60 mM K+. This method is the first to directly sample multiple neurotransmitters from perfused, isolated retinas and to observe changes in efflux of these neurotransmitters as a result of pharmacological stimulation.

Anatomical and electrophysiological evidence for GABAergic bipolar cells in tiger salamander retina

Our previous work showed that about 12% of bipolar cells in salamander retina synthesize and take up gamma-aminobutyric acid (GABA), are GABA transporter (GAT)-immunoreactive, and respond with a GAT current to extracellularly applied GABA, suggesting that these bipolar cells use GABA, in addition to glutamate, as a neurotransmitter. Further support for this idea was obtained in this study by use of immunogold electron microscopy and whole-cell patch clamp electrophysiology. Ultrastructural analysis showed that amacrine cell and ganglion cell processes were postsynaptic to GABA-immunoreactive synapses made by bipolar cell axon terminals. Whole-cell recordings were obtained from amacrine and ganglion cells in response to activation of bipolar cells by puffing KCl at their dendrites in the outer plexiform layer. Inhibitory postsynaptic currents were observed in several third order neurons, even after blocking the excitatory postsynaptic responses, generated in the inner plexiform layer, with a combined application of NMDA and non-NMDA receptor antagonists, AP-5 and CNQX. These ultrastructural and electrophysiological data support our previous neurochemical results, and suggest that the retinal through-information pathway in salamander includes both inhibitory GABAergic as well as excitatory glutamatergic synaptic mechanisms.

Effects of L-serine on neurons in vitro

We have examined the influence of amino acids on chicken embryonic dorsal root ganglion (DRG) neurons in vitro and have found that serine has a marked impact on the morphology of the developing neurites. In cultures supplemented with L-serine (but not D-serine) at micromolar concentrations (10-200 microM), the length of the neurites was increased by up to 100% and they were seen to develop a more complex branching pattern. These effects of L-serine were found to be concentration-dependent and stereospecific and were observed on several different substrata such as laminin, Ng-CAM and axonin-1. Similar observations were also made in the case of embryonic retinal explants, while the addition of non-essential amino acids other than L-serine to DRG neurons was found to have no effect. We conclude that, although belonging to the group of non-essential amino acids and not a recognized neurotransmitter, L-serine is an important factor for the morphological differentiation of neurons in vitro.

Comparison of the actions of glycine and related amino acids on isolated third order neurons from the tiger salamander retina

Whole cell voltage and current clamp recordings were obtained from third order neurons isolated from the salamander retina. Using cross desensitization, the structure-function relationship of short chain amino acids on the glycine receptor were examined. L-Serine, L-alanine, beta-alanine and taurine all cross desensitized with glycine, but did not show significant cross desensitization with GABA. This indicates that these amino acids act at the glycine receptor. The order of potency was glycine >> beta-alanine > taurine >> L-alanine > L-serine. TAG, a reputed selective taurine antagonist, was equally effective in blocking taurine and glycine currents. There is no evidence for distinct receptors for taurine. Amino acids with larger moieties at the alpha carbon, such as threonine and valine, produced inactive ligands. Placing a methyl group on the amine of glycine or esterification of the carboxyl group also greatly reduced activity. Based on these modifications of the glycine molecule, it appears that selectivity at the glycine receptor results in part from steric restrictions at all three sites in the glycine chain. The steric interference is most critical at the carboxyl and amino ends, and less limiting at the alpha carbon. Doses of L-serine that had only slight effects in voltage clamp experiments, nevertheless produced large effects in current clamp experiments. This indicates that several endogenous amino acids can have significant effects on membrane voltage, even when their shunting activity may be small. High concentrations of agonists produced desensitization in the voltage clamp records, but there was little evidence of desensitization in the current clamp experiments. These results indicate that several endogenous amino acids can activate the glycine receptor, but there is no evidence for a discrete receptor for taurine, beta-alanine, L-alanine or L-serine. Since all these endogenous amino acids have similar amino and acid terminals, reduction in potency results from steric interference around the alpha carbon. This graded potency may have functional significance in mediating inhibition.

The post-transurethral resection of prostate syndrome: therapeutic proposals

Absorption of large volumes of irrigation fluid during transurethral resection of the prostate (TURP) can produce hyponatremia, coma, blindness, and cardiorespiratory depression. This has been termed the "post-TURP syndrome." The pathophysiology and management of this syndrome are controversial. A case of severe post-TURP syndrome due to the absorption of hypotonic glycine irrigant is reported. This patient developed severe hyponatremia, slight hypo-osmolality, hyperglycinemia, hyperserinemia, hyperammonemia, and hypocalcemia. He became comatose, profoundly hypotensive, bradycardic, and transiently blind. Because of this combination of severe biochemical, neurologic, and hemodynamic abnormalities, we elected to treat this patient with hemodialysis. The biochemical and clinical manifestations resolved completely. The pathophysiology and treatment of this disorder are discussed and a role for hemodialysis is suggested.