Non-exocytotic GABA overflow in rat striatum inhibits gnawing

In vivo monitoring of amine neurotransmitters using microdialysis with on-line capillary electrophoresis

Microdialysis sampling was coupled via a flow-gated interface on-line to capillary electrophoresis with laser-induced fluorescence (LIF) detection for in vivo monitoring of neuroactive amino acids and amines. In the instrument, analytes are derivatized precolumn with o-phthaldehyde and beta-mercaptoethanol to form fluorescent isoindole products. The instrument was improved over previous designs by incorporating a sheath-flow cuvette for reduced background in LIF detection which improved sensitivity by 15-fold. The methodology was improved by utilizing a voltage ramped injection which allowed generation of 500000 theoretical plates with 20 s separations. Resolution of the isoindole derivatives was further improved by addition of hydroxypropyl-modified beta-cyclodextrin to the electrophoresis buffer. The new instrumentation and methods allow resolution and detection of glutamate, gamma-aminobutyric acid, glycine, aspartate, serine, taurine, glutamine and dopamine (if levels are elevated) collected from in vivo sampling probes every 20 s. The technique is suited to continuous monitoring for dynamic measurements of these compounds in vivo.

GABA transaminase inhibition induces spontaneous and enhances depolarization-evoked GABA efflux via reversal of the GABA transporter

The GABA transporter can reverse with depolarization, causing nonvesicular GABA release. However, this is thought to occur only under pathological conditions. Patch-clamp recordings were made from rat hippocampal neurons in primary cell cultures. Inhibition of GABA transaminase with the anticonvulsant gamma-vinyl GABA (vigabatrin; 0.05-100 microm) resulted in a large leak current that was blocked by bicuculline (50 microm). This leak current occurred in the absence of extracellular calcium and was blocked by the GABA transporter antagonist SKF-89976a (5 microm). These results indicate that vigabatrin induces spontaneous GABA efflux from neighboring cells via reversal of GABA transporters, subsequently leading to the stimulation of GABA(A) receptors on the recorded neuron. The leak current increased slowly over 4 d of treatment with 100 microm vigabatrin, at which time it reached an equivalent conductance of 9.0 +/- 4.9 nS. Blockade of glutamic acid decarboxylase with semicarbazide (2 mm) decreased the leak current that was induced by vigabatrin by 47%. In untreated cells, carrier-mediated GABA efflux did not occur spontaneously but was induced by an increase in [K(+)](o) from 3 to as little as 6 mm. Vigabatrin enhanced this depolarization-evoked nonvesicular GABA release and also enhanced the heteroexchange release of GABA induced by nipecotate. Thus, the GABA transporter normally operates near its equilibrium and can be easily induced to reverse by an increase in cytosolic [GABA] or mild depolarization. We propose that this transporter-mediated nonvesicular GABA release plays an important role in neuronal inhibition under both physiological and pathophysiological conditions and is the target of some anticonvulsants.

Chronic exercise alters caudal hypothalamic regulation of the cardiovascular system in hypertensive rats

Previous studies have documented a deficit in the GABA neurotransmitter system within the caudal hypothalamus (CH) of spontaneously hypertensive rats (SHR). The reduction in inhibitory influence on this cardiovascular excitatory brain region is associated with an increased neuronal activity and resting blood pressure. The purpose of this study was to determine if chronic treadmill and wheel-running activities alter the ability of the CH to regulate cardiovascular function. SHR were exercised on a treadmill (5 times/wk) at moderate intensity or allowed free access to running wheels (7 days/wk) for a period of 10 wk. Resting blood pressures were obtained before and after the exercise training periods. After the exercise period, rats were anesthetized and microinjection experiments were performed. Treadmill-trained SHR exhibited a significantly blunted developmental rise in resting blood pressure after 10 wk of exercise. A similar yet less marked effect was observed in wheel-run rats. Microinjection of the GABA synthesis inhibitor 3-mercaptopropionic acid (3-MP) into the CH of nonexercised SHR did not produce any change in arterial pressure. In contrast, microinjection of 3-MP into the CH produced significant increases in blood pressure and heart rate in exercised SHR. These results demonstrate that exercise training can alter CH cardiovascular regulation in hypertensive rats and therefore may play a role in increasing cardiovascular health.

Determination of striatal extracellular gamma-aminobutyric acid in non-hibernating and hibernating arctic ground squirrels using quantitative microdialysis

This study determined extracellular concentrations of gamma-aminobutyric acid ([GABA](ecf)) in striatum of non-hibernating and hibernating arctic ground squirrels to test the hypothesis that an increase in [GABA](ecf) was associated with profound CNS depression during hibernation. Quantitative microdialysis procedures were employed to circumvent the effects of low temperature on the relative recovery of the analyte across the dialysis membrane and yielded for the first time quantitative in vivo estimates of [GABA](ecf) in any brain region or any species. Laboratory housed, wild caught Arctic ground squirrels (Spermophilus parryii) were implanted intraperitoneally with radio transmitters that enabled the telemetric monitoring of activity and core body temperature (T(b)) and bilaterally implanted with cranial guide tubes that enabled the implantation of microdialysis probes into the striatum. Striatal [GABA](ecf) was determined in unrestrained, non-hibernating ground squirrels (T(b) range 34.7-38.9 degrees C) and hibernating ground squirrels (T(b) range 2.9-3.9 degrees C) using extrapolation to zero flow and very slow flow microdialysis techniques. The results show that [GABA](ecf) in non-hibernating squirrels was 73 nM and this level was decreased by approximately 50% during hibernation thereby suggesting that an increase in [GABA](ecf) does not play a major role in CNS depression during hibernation. The reduction of [GABA](ecf) parallels a decrease in plasma and CSF [glucose] and may be related to a decrease in GABA synthesis or reduced voltage dependent release. This paper demonstrates that measurement of extracellular concentrations of neurotransmitters in animals with vastly different body temperatures is possible using microdialysis techniques of extrapolation to zero flow or very slow flow rates that enable 100% recovery. Such quantitative techniques may prove valuable in the study of the neurochemistry of the cerebral mechanisms of hibernation and tolerance to cerebral ischemia exhibited by hibernating animals.

An ultrastructural analysis of tissue surrounding a microdialysis probe

Microdialysis is a widely used in vivo sampling technique commonly used to monitor extracellular levels of a variety of molecules including neurotransmitters and metabolites. To facilitate interpretation of microdialysis results, this study critically examines changes in synaptic morphology induced by microdialysis. Tissue surrounding microdialysis probes was examined using light and electron microscopy at three distances from the probe tract. Microdialysis probes were implanted into rat striatum, and after 40 h of post-operative recovery were perfused with a modified Ringer's solution. Light microscope analysis revealed tissue disruption up to 1.4 mm from the probe site. Axonal damage indicative of non-excitotoxic insult was also seen as far away from the probe as was examined. The presence of dark-degenerating neurons was also noted and estimates of neuronal densities revealed loss up to 400 microm from the probe tract. This study, the first qualitative ultrastructural investigation of neuropil surrounding the probe site, indicated swollen processes up to 1.4 mm from the probe tract. Swollen mitochondria and bloated endoplasmic reticulum suggest intracellular chemical disruption. Tissue damage resulting in synaptic and neuronal disruption may affect neurotransmitter efflux or extracellular concentrations of metabolites.