Depletion of Synaptosomal Neurotransmitter Pool by Sudden Cooling

Static and dynamic discharge properties of vestibular-nerve afferents in the mouse are affected by core body temperature

The goal of this study was to determine the effect of changes in core body temperature on the resting discharge rate and sensitivity of vestibular-nerve afferents. Extracellular recordings were made from vestibular-nerve afferents innervating the semicircular canals in anesthetized C57BL/6 mice maintained at a core body temperature of either 30-32 degrees C (T (31)) or 35-37 degrees C (T (36)). The resting rates of regular (CV* < 0.1) and irregular afferents (CV* > 0.1) were lower at T (31) than at T (36). Sensitivity and phase were compared for rotations ranging from 0.1 to 12 Hz by calculating coefficients of a transfer function, g . t(c)S . (t(z)S +1)/(t(c)S + 1), for each afferent. The sensitivity (g) increased with CV* and with higher core body temperature. The value of the coefficient representing the low-frequency dynamics (t (c)) varied inversely with CV* but did not change with core body temperature. The high-frequency dynamics represented by t (z) increased with CV* and decreased with higher core body temperature. These findings indicate that changes in temperature have effects on the static and dynamic properties of vestibular-nerve afferents.

Spinal seizures evoked by sudden cooling of amphibian isolated spinal cords: involvement of excitatory amino acids

Sudden cooling of the isolated spinal cord of frogs results in characteristic seizure-like activity in the hind legs. In the present investigation, these spinal seizures induced by sudden cooling (SSSC) were studied to determine whether excitatory amino acids (EAAs) are involved in the mediation of this activity. The nonspecific EAA antagonist, L-glutamic acid diethyl ester and cis-2,3-piperidine dicarboxylic acid inhibited the clonic and tonic phase of SSSC after intralymphatic or intrathecal administration. The antagonist gamma-D-glutamylaminomethylsulfonic acid and gamma-D-glutamyltaurine also suppressed both phases after intrathecal injections. The NMDA receptor antagonist DL-2-amino-5-phosphonovaleric acid, DL-2-amino-7-phosphonoheptanoic acid, and 3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid were effective inhibitors of the tonic phase and actually prolonged the duration of the clonic phase, an effect similar to that observed after low doses of gamma-D-glutamylglycine. SSSC were resistant to spinal perfusion to tetrodotoxin (1 microM). The concentrations of glutamate, aspartate, and glycine were increased in the Ringer's solution surrounding rapidly cooled spinal cord slices, but only in cords from species that elicited some magnitude of SSSC, not in cords from species resistant to induction of SSSC. Our data support the hypothesis that EAAs play a role in SSSC via activation of quisqualate receptors.

Compartmentation and release of exogenous GABA in sheep brain synaptosomes

Exogenous tritiated gamma-aminobutiric acid ([3H]GABA) is retained in two compartments in sheep cortex synaptosomes, corresponding to cytoplasmic and vesicular spaces, assuming that freeze-thawing the synaptosomes loaded with [3H]GABA releases the cytoplasmic [3H]GABA (81 +/- 3.9%), and that subsequent solubilization of the synaptosomes with 1% sodium cholate releases the vesicular [3H]GABA (19 +/- 3.9%). Depolarization of synaptosomes with 40 mM K+ in a Na+-medium, in the absence of Ca2+, releases 20.3 +/- 2.7% of the [3H]GABA retained in the synaptosomes. The [3H]GABA released under these conditions comes predominantly from the cytoplasm. The presence of 1 mM Ca2+ during depolarization releases an additional 13% (a total of about 33.5 +/- 9.9%) of the releasable [3H]GABA, and the [3H]GABA release which is Ca2+-dependent also comes mostly from the cytoplasmic compartment. When choline replaces external Na+, the [3H]GABA release is absolutely Ca2+-dependent, and the [3H]GABA released also comes mostly from the cytoplasmic pool. Therefore, it appears that [3H]GABA taken up by synaptosomes is accumulated mostly in the cytoplasmic compartment from which it is released upon depolarization. The technique described permits distinguishing the effect of different factors on the two pools of accumulated [3H]GABA.

Fusion of secretory vesicles isolated from rat liver

Secretory vesicles isolated from rat liver were found to fuse after exposure to Ca2+. Vesicle fusion is characterized by the occurrence of twinned vesicles with a continuous cleavage plane between two vesicles in freeze-fracture electron microscopy. The number of fused vesicles increases with increasing Ca2+-concentrations and is half maximal around 10(-6)M. Other divalent cations (Ba2+, Sr2+, and Mg2+) were ineffective. Mg2+ inhibits Ca2+-induced fusion. Therefore, the fusion of secretory vesicles in vitro is Ca2+ specific and exhibits properties similar to the exocytotic process of various secretory cells. Various substances affecting secretion in vivo (microtubular inhibitors, local anesthetics, ionophores) were tested for their effect on membrane fusion in our system. The fusion of isolated secretory vesicles from liver was found to differ from that of pure phospholipid membranes in its temperature dependence, in its much lower requirement for Ca2+, and in its Ca2+-specificity. Chemical and enzymatic modifcations of the vesicle membrane indicate that glycoproteins may account for these differences.

Effects of pH changes and charge characteristics in the uptake of norepinephrine by synaptosomes of rat brain

The pH dependence of the initial uptake of norepinephrine by rat whole brain synaptosomes was studied using short incubation times at 37 degrees C in order to examine the possible involvement of the phenolic OH group. The pH vs. uptake profile exhibits a maximum near pH 8.2 in H2O medium. When the medium was changed to 2H2O, the profile showed a shift of maximum corresponding to the pKa change of the phenolic OH group. The pH vs. uptake profile of tyramine was quite different from that of norepinephrine. These pH effects on uptake were explained as manifestations of the involvement of the phenolic OH group in the process. The amine and phenolic hydroxyl groups in norepinephrine were studied separately by employing two series of compounds structurally related to catecholamines, amphetamine-like and catechol-like, for their inhibitory effects on the uptake. The inhibitions were affected by changes in pH with changes in opposite directions found for the two series indicating the need for a positive charge in the side chain and suggesting an effect of the negative charge on the ring. These charge characteristics agreed with the pH profile observed in uptake. Consequently, the two groups with opposite charge characteristics in norepinephrine both appear to function in the uptake process.

Uptake and release of calcium by rat brain synaptosomes

Rat brain synaptosomes, prepared by discontinuous Ficoll density gradient centrifugation, accumulated 45Ca during brief incubations in modified Krebs-Ringer media. Uptake of 45Ca was increased by 5 mM glutamate and 50 mM KCl, conditions that depolarize nerve cells; uptake of 22Na was also increased by these agents. With 0.2 mM diphenylhydantoin, the increased 45Ca uptake due to KCl was diminished, whereas that due to glutamate was less affected; conversely, with 0.15 muM tetrodotoxin the increased 45Ca uptake due to glutamate was diminished, whereas that due to KCl was less affected. Both diphenylhydantoin and tetrodotoxin diminished the augmented uptake of 22Na due to KCl and glutamate; thus the increased uptake of 45Ca under depolarizing conditions may be dissociated from the increased influx of sodium. Ruthenium red decreased the uptake of 45Ca under all conditions, as did procaine and the lanthanide Pr3+. Neither 5 mM glutamate nor 50 mM KCl increased 45Ca uptake by brain mitochondria under comparable experimental conditions, whereas ATP increased the uptake by mitochondria but not that by these synaptosomes. Altering the sodium gradient by equimolar substitution of lithium or choline for sodium in the medium increased 45Ca uptake, whereas 22Na uptake was decreased. Inhibiting the sodium pump by ouabain or strophanthidin also increased 45Ca uptake, and increased 22Na uptake as well. The increased uptake of 45Ca induced by ouabain was inhibited by diphenylhydantoin and tetrodotoxin. Measurements of the total calcium content showed that conditions producing an increased uptake of 45Ca also produced a net uptake of calcium, rather than merely accelerating a 45Ca-40Ca exchange. Experiments measuring the loss of previously accumulated 45Ca showed that directly decreasing the sodium gradient or inhibiting the sodium pump slowed the loss of 45Ca. These data are considered in terms of calcium influx through 'leak' pathways and gated channels (sensitive to membrane depolarization) and of net efflux dependent on a coupled sodium-calcium exchange mechanism.

Decrease of uptake and exchange of neurotransmitter amino acids after depletion of their synaptosomal pools

Synaptosomes prelabeled at 37 degrees C with radioactive amino acids (GABA, glutamate, glycine, taurine, alpha-aminoisobutyric acid, phenylalanine, leucine) and then washed at 0 degrees C on Millipore filters (DAWP 02500) lost 60-70% of the accumulated radioactivity. The loss was similar with exogenous tritiated GABA and glutamate, and with [14C]GABA and [14C]glutamate metabolically derived from [14C]glucose. In contrast, radioactive norepinephrine, dopamine and 5-hydroxytryptamine were almost totally retained by cold shocked synaptosomes. After pretreatment with reserpine and nialamide the loss of norepinephrine became significantly greater (about 25%). The uptake of radioactive GABA, glutamate and clycine after cold shock was about 50% reduced, whereas that of radioactive biogenic amines was less affected (reduction of 22% for norepinephrine, 29% for 5-hydroxytryptamine and 35% for dopamine). The loss of amino acids and the reduction of uptake could be minimized by performing the cold shock in hypertonic conditions. In synaptosomes prelabeled with [3H]GABA, a good correlation was observed among magnitude of amino acid pool depletion induced by cold shock or by 56 mM KCl, decrease of subsequent accumulation of [14C]GABA, and decrease of [14C]-GABA-stimulated [3H]GABA release (homoexchange).