Robert W. P. Cutler

Effect of barbiturates on release of endogenous amino acids from rat cortex slices

The potassium-stimulated, calcium-dependent release of endogenous GABA and glutamic acid was suppressed by pentobarbital. The ouabain and veratridinestimulated fluxes of the amino acids, calcium-independent processes, were not suppressed by pentobarbital. Release of GABA and glutamic acid was not suppressed by pentobarbital in the presence of the calcium ionophore A23187. Of eight barbiturates studied at equimolar concentrations six were found to inhibit GABA release. Thiopental was the most potent, and phenobarbital and secobarbital were inactive.

The effect of penicillin on the release of γ-aminobutyric acid from cerebral cortex slices

Penicillin is known to block the depressant action of GABA on cortical and spinal neurons. In this study we examined the effect of penicillin on the release of endogenous amino acids from rat cerebral cortex slices. The principal finding was that penicillin blocked the potassium-stimulated release of GABA, a calcium-dependent process. It did not block the veratridine-stimulated release of GABA, a calcium-independent process. It did not affect the release of other putative transmitter amino acids. Inhibition of GABA release from nerve endings may explain in part the ability of penicillin to depress synaptic inhibitory mechanisms.

The release of endogenous amino acids into the vitreous of the intact eye of the albino rat: effect of light, potassium, and ouabain

The vitreal space of the intact eye of albino rats was perfused in vivo. The concentration of several endogenous amino acids in the vitreal effluent was measured by the [3H]microdansylation procedure. GABA was never detected despite a sensitivity of the method of 0.5 pmol. In contrast to previous results obtained in pigmented rats, photic stimulation with flashing white light did not alter the release of glycine or any of the other amino acids. Potassium (60 mM) and ouabain (0.1 mM) evoked a specific release of glycine. The potassium-evoked release was blocked by magnesium suggesting a neuronal site of origin of glycine. Ouabain-evoked release was not blocked by magnesium. The results were contrasted with experiments on radiolabeled amino acid release from retinas preloaded and superfused in vitro, a condition in which glial localization of exogenous amino acids predominates.

Inhibition of gamma-aminobutyric acid release from rat cerebral cortex slices by barbiturate anesthesia.

Amobarbital and pentobarbital anesthesia inhibited the potassium-stimulated, Ca-dependent release of γ-aminobutyric acid (GABA) from rat cerebral cortex slices during incubation in vitro. Inhibition of GABA release was not found when slices were prepared from rats shortly after they awakened from amobarbital anesthesia. Phenobarbital anesthesia did not affect the release of GABA.

Bidirectional movement of γ-aminobutyric acid in cerebral cortex slices incubated in small volumes of medium

Rat cerebral cortex slices were incubated in small volumes of medium in order to study the bidirectional movements of gamma-aminobutyric acid (GABA) between tissue and medium, and to measure the extent of release of endogenous amino acids of interest as potential neurotransmitters. The apparent uptake, real uptake, and net change of GABA were calculated from the changes in [14C]GABA and endogenous GABA in the medium. The extent of release of endogenous amino acids was dependent upon the amino acid the duration of incubation, and the volume of incubating medium. Substantial amounts of endogenous amino acids may accumulate in the medium under conditions commonly employed in amino acid uptake studies. The accumulation of endogenous GABA may affect the calculation of GABA uptake when this is based on the initial specific activity of exogenous [14C]GABA. The difference between apparent uptake and real uptake was particularly prominent in the presence of the depolarizing agents, potassium or veratridine, which caused a marked increase in GABA release. Exchange or net release of GABA was found when slices were incubated briefly in media containing 1.3 micrometer or 12.5 micrometer exogenous GABA; net uptake accounted for 50% of observed uptake when slices were incubated in media containing 25 micrometer GABA.

Maturational changes of amino acid concentration in cerebrospinal fluid of the rat.

The cisternal spinal fluid (CF), plasma, and brain stem concentrations of 6 amino acids were measured in suckling rats between birth and one months of age. Each of the amino acids studied had its distinctive maturational pattern of CF concentration. Presumably these amino acids have separate mechanisms of transport from CF, which mature at different times.

Neurochemical Aspects of Blood-Brain-Cerebrospinal Fluid Barriers

The blood-brain barrier and the blood-cerebrospinal fluid (CSF) barrier are terms that are deeply entrenched in the literature of neurobiology. The concept of a permeability barrier between the blood and the nervous system evolved from observations of Ehrlich that the brain did not become stained after intravenous injection of aniline dyes (e.g., trypan blue). Goldmann later showed that the brain became stained after injection of trypan blue into the CSF. For many years, the barrier was considered to be an anatomical one that absolutely restricted the passage of certain substances into the brain. When tracer methodology became available, it became clear that most substances were not absolutely restricted from the brain, but rather that their rates of entry or ultimate ability to accumulate were reduced with respect to other organs. In this way, the chemical environment of the brain could be carefully guarded and exquisitely regulated. This chapter provides a brief account of the mechanisms that contribute to the regulation of concentrations of various classes of solutes in the brain and CSF. It seems likely that no fundamental differences exist in the mechanisms of transport at the blood-brain and blood-CSF interfaces. Several monographs expand the subject for interested readers.11,35,48,52,53,81

Release of Amino Acids from the Spinal Cord in Vitro and in Vivo

In the last decade, considerable evidence has supported the initial suggestion by Aprison and Werman2 that glycine is a likely inhibitory transmitter in the mammalian spinal cord. The concentration of glycine becomes progressively higher in more caudal areas of the nervous system and it is particularly high, on the order of 5 µmoles/gram, in regions of spinal cord known to contain inhibitory interneurons. The concentration falls after partial spinal cord ischemia in a manner proportional to the loss of interneurons. 12 The action of glycine is antagonized by strychnine, which is known to block postsynaptic inhibitory mechanisms.5 There is reason to believe that other amino acids may have roles as neurotransmitters in the spinal cord, based on less complete evidence than that for glycine. For example, GABA has been proposed as a presynaptic inhibitory transmitter at primary afferent terminals. It is found in high concentrations in the dorsal horn, and local ischemia of this region leads to a marked fall in concentration.23 Deafferentation of the spinal cord by dorsal root section leads to a reduction in glutamic acid decarboxylase activity in the dorsal lateral region of cord, together with a fall in GABA content and a reduction of [3H] GABA uptake.14

Effect of penicillin, pentylenetetrazol and strychnine on the release of endogenous amino acids into the vitreous of the intact eye of the rat

Abstract The vitreous of the intact eye of the albino rat was perfused in vivo and the concentration of amino acids was measured in the vitreal effluent under various conditions. Elevated potassium concentration in the medium stimulated the release of glycine and taurine. Penicillin inhibited the K+-evoked release of both amino acids; pentylenetetrazol inhibited the release of only taurine; and strychnine did not inhibit the release of either amino acid. Penicillin did not inhibit the ouabain-evoked release of glycine. Part of the convulsant action of penicillin may reflect its ability to lower the concentration of inhibitory amino acids in the extracellular fluid of the nervous system under calcium-dependent depolarizing conditions.