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 characterize gamma-aminobutyric acid (GABA) release and the effects induced by a preceding neuron-depleting ibotenic acid (IBO) lesion in the rat caudate-putamen (CPu). Extracellular GABA overflow was monitored in the intact and excitotoxically lesioned CPu, either 7-10 days (acute) or more than 3 months post-lesioning (chronic), using loop type dialysis probes perfused at a rate of 2 microliters/min. In the intact CPuu, basal GABA levels were 0.97 pmol/30 microliters of dialysate in the awake animals and 0.76 pmol/30 microliters under halothane anaesthesia. In both the acute and chronic IBO lesioned CPu the extracellular GABA levels were reduced by 80% and 67%, respectively, under halothane anaesthesia. KCl added to the perfusion fluid at a concentration of 100 mM resulted in dramatic increases in GABA overflow from baseline levels in the intact CPu (60- to 70-fold), which were almost totally abolished (> 95%) in the excitotoxically lesioned CPu. Veratridine administered at 75 microM, produced a 45-fold 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 mM), 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 Ca(2+)-free conditions (with 20 mM Mg2+ added) or tetrodotoxin (TTX, 1 microM) did not alter the basal GABA overflow in the intact CPU under halothane anaesthesia, the omission of Ca2+ resulted in a 47% reduction in basal extracellular GABA levels in awake, freely moving animals. Nipecotic acid-induced GABA overflow was reduced by 22% under Ca(2+)-free conditions, and by 33% in the presence of 1 microM TTX. Moreover, KCl-evoked GABA overflow was reduced by 86% in Ca(2+)-free conditions and by 40% when administered in the presence of 1 microM TTX. These results indicate that the extracellular GABA levels recorded by intracerebral microdialysis in the CPu are derived predominantly from neuronal sources. Under baseline resting conditions only a small fraction (up to 20-30%) of the neuronal release was Ca(2+)-dependent and TTX-sensitive (i.e. possessing the characteristics of impulse-dependent vesicular release).(ABSTRACT TRUNCATED AT 400 WORDS)
Campbell
K
K
Department of Medical Cell Research, University of Lund, Sweden.
Kalén
P
P
Lundberg
C
C
Wictorin
K
K
Rosengren
E
E
Björklund
A
A
D000818:Animals; D002118:Calcium / Q000502:physiology; D002421:Caudate Nucleus / Q000166:cytology / Q000378:metabolism*; D002851:Chromatography, High Pressure Liquid; D003956:Dialysis; D004563:Electrochemistry; D005110:Extracellular Space / Q000378:metabolism; D005260:Female; D007051:Ibotenic Acid / Q000494:pharmacology; D009457:Neuroglia / Q000378:metabolism*; D009474:Neurons / Q000378:metabolism*; D009557:Nipecotic Acids / Q000494:pharmacology; D011189:Potassium Chloride / Q000494:pharmacology; D011392:Proline / Q000031:analogs & derivatives*; D011699:Putamen / Q000166:cytology / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D013779:Tetrodotoxin / Q000494:pharmacology; D014701:Veratridine / Q000494:pharmacology; D005680:gamma-Aminobutyric Acid / Q000378:metabolism*
D009557:Nipecotic Acids; C030278:nipecotic acid; D007051:Ibotenic Acid; D013779:Tetrodotoxin; D005680:gamma-Aminobutyric Acid; D011189:Potassium Chloride; D014701:Veratridine; D011392:Proline; C010983:homoproline; D002118:Calcium