Efflux of GABA, glutamate and glutamine from the brain: A mass fragmentographic Study

Pathogenic role of glutamate in hyperthermia-induced seizures

Hyperthermia induces seizures in both humans and rodents, but the underlying mechanism remains unknown. The present study showed that hyperthermia, causing rapid increase in body temperature, increases the concentration of glutamate (Glu) released into a cortical perfusate before onset of seizures in rats and that this increase in Glu concentration correlated with a decrease in seizure threshold temperature. These results indicate that increased cortical extracellular Glu induced by hyperthermia contributes to onset of seizures. The same mechanism may be involved in clinical seizures induced by fever in patients with febrile convulsions or epilepsy.

Benzodiazepine binding sites in rat submaxillary gland: absence of markers of GABA system

Specific binding of [3H]diazepam to the nuclear, mitochondrial and microsomal subcellular fractions of the rat submaxillary gland is reported. Apparent KD values were similar in the three fractions studied (around 80 nM). The nuclear and mitochondrial fractions contain 95% of the total sites detected (20 pmoles/mg prot.). The sites exhibit a much higher affinity for Ro 5-4864 than for clonazepam or Ro 15-1788, and are not modified by 10(-6) to 10(-4) GABA or 10(-5) M bicuculline. Neither sympathetic denervation or parasympathetic decentralization modified [3H]Dz binding, which is found to be considerably lower in the early post-natal state as well as following duct ligation of the gland of adult rats. The results suggest that these peripheral Bz binding sites are mainly located in the acini. No specific GABA binding or GAD activity was detected either in the presence or absence of NaCl. Very low levels of endogenous GABA (58 +/- 3 nmoles/g tissue) were found in homogenates of the gland.

In vitro changes in gamma-aminobutyric acid output from the cerebral cortex induced by inhibitors of gamma-aminobutyric acid uptake and metabolism

The effects of inhibitors of gamma-aminobutyric acid (GABA) metabolism or uptake on GABA output from the cerebral cortex was studied by means of a collecting cup placed on the exposed cortex of rats anaesthetized with urethane. GABA was identified and quantified by a mass-fragmentographic method. Ethanolamine-O-sulphate (10(-2) M) applied directly on the cerebral cortex caused a long-lasting twofold increase in GABA output, whereas DL-2,4-diaminobutyric acid (5 X 10(-3) M) caused a sevenfold increase and beta-alanine was active. The results indicate that glial uptake has little effect on GABA inactivation in the cerebral cortex. The inhibition of neuronal uptake seems a more effective tool to increase GABA concentration in the synaptic cleft, and consequently also in GABA output, than the inhibition of GABA metabolism.

The release of endogenous GABA and glutamate from the cerebral cortex in the rat

1. The release of endogenous GABA and glutamate from the cerebral cortex was measured using a cortical cup technique in unanaesthetized freely moving rats and anaesthetized rats by means of a sensitive and specific mass-spectrometric procedure. 2. GABA release was not affected by the presence of the dura mater or by anaesthesia. Glutamate output was reduced by urethane but not by pentobarbital anaesthesia and by the presence of the dura. 3. An isotonic solution containing 50 mM KCl placed epidurally within the cup elicited a significant short-lasting increase in glutamate output, a decrease in GABA output and a short-lasting electrocorticogram (ECoG) activation. 4. When the dura was removed, a high K+ solution placed on the exposed cerebral cortex elicited a 7--8 fold increase in GABA output accompanied by a marked decrease in glutamate output and by ECoG synchronization. The changes in GABA and glutamate output had parallel time-course and were prevented by the application within the cup of tetrodotoxin (3 X 10(-5) M). 5. Amphetamine at the doses of 3.7 and 7.4 mumol . kg-1 i.v. increased glutamate output and at the dose of 37 mumol . kg-1 i.v. increased GABA output. Both effects were prevented or reduced by haloperidol pretreatment (0.65 mumol . kg-1 i.v.). 6. It is concluded that GABA and glutamate released from the cerebral cortex and diffused into an epidural or cortical cup originate at least in part from the brain. The rate of their release is influenced by changes in neuronal activity. The measurement of their rate of release offers a useful tool for the study of the functional role of cortical GABA and glutamate-releasing neurons.