EXTRACELLULAR GAMMA-AMINOBUTYRIC-ACID LEVELS IN THE RAT CAUDATE-PUTAMEN - MONITORING THE NEURONAL AND GLIAL CONTRIBUTION BY INTRACEREBRAL MICRODIALYSIS

Intracerebral microdialysis with high pressure liquid chromatography ( HPLC) coupled to electrochemical detection was employed to characteriz e gamma-aminobutyric acid (GABA) release and the effects induced by a preceding neuron-depleting ibotenic acid (IBO) lesion in the rat cauda te-putamen (CPu). Extracellular GA.BA overflow was monitored in the in tact and excitotoxically lesioned CPu, either 7-10 days (acute) or mor e than 3 months post-lesioning (chronic), using loop type dialysis pro bes perfused at a rate of 2 mul/min. In the intact CPuu, basal GABA le vels were 0.97 pmol/30 mul of dialysate in the awake animals and 0. 76 pmol/30 mul under halothane anaesthesia. In both the acute and chroni c IBO lesioned CPu the extracellular GABA levels were reduced by 80% a nd 67%, respectively, under halothane anaesthesia. KCl added to the pe rfusion fluid at a concentration of 100 mM resulted in dramatic increa ses in GABA overflow from baseline levels in the intact CPu (60- to 70 -fold), which were almost totally abolished (> 95%) in the excitotoxic ally lesioned CPu. Veratridine administered at 75 muM, produced a 45-f old increase in GABA overflow in the intact CPu, but failed to produce any effect in the lesioned CPu. The addition of nipecotic acid (0.5 m M), a GABA uptake blocker, increased basal extracellular GABA levels 6 -15-fold in the intact CPu, while GABA overflow in either the acute or chronic lesioned CPu was not significantly altered. Although Ca2+-fre e conditions (with 20 mM Mg2+ added) or tetrodotoxin (TTX, 1 muM) did not alter the basal GABA overflow in the intact CPU under halothane an aesthesia, the omission of Ca2+ resulted in a 47% reduction in basal e xtracellular GABA levels in awake, freely moving animals. Nipecotic ac id-induced GABA overflow was reduced by 22% under Ca2+-free conditions , and by 33% in the presence of 1 muM TTX. Moreover, KCl-evoked GABA o verflow was reduced by 86% in Ca2+-free conditions and by 40% when adm inistered in the presence of 1 muM TTX. These results indicate that th e extracellular GABA levels recorded by intracerebral microdialysis in the CPu are derived predominantly from neuronal sources. Under baseli ne resting conditions only a small fraction (up to 20-30%) of the neur onal release was Ca2+-dependent and TTX-sensitive (i.e. possessing the characteristics of impulse-dependent vesicular release). While glial contributions (as assessed from the excitotoxic lesioned CPu), may acc ount for as much as 20-40% of the basal extracellular GABA levels, mor e than 95% of the evoked release (i.e. after application of KC] and ve ratridine) is neuronal in origin. The KCl-evoked GABA release was like ly vesicular in nature since removal of Ca2+ blocked the response by a lmost 90%. The reductions in basal and evoked extracellular GABA level s seen in the excitotoxically lesioned CPu persisted also after long s urvival periods, suggesting that functional recovery of GABA release d oes not occur.