Pharmacological heterogeneity of gamma-aminobutyric acid receptors during development suggests distinct classes of rat cerebellar granule cells in situ.
The gamma-aminobutyric acid receptor (GABA(A)R) represents a ligand-gated Cl(-)-channel assembling as heteropentamere from 19 known subunits. Cerebellar granule cells contain a unique subset, namely the alpha1-, alpha6-, beta2-, gamma2- and delta-subunits. We studied their GABAergic pharmacology in situ using whole-cell patch-clamp recordings in brain slices and a modified Y-tube application system. The distribution of the EC50s for GABA in young (P8-P14) and medium aged animals (P15-P28) could be fitted with the sum of two Gaussian distributions with means of 60 and 185 microM and 27 and 214 microM, respectively. In older animals (P29-P48) the observed homogeneous range of sensitivities fitted a single Gaussian distribution (11 microM). In young animals (< or =P14) GABA-responses were largely insensitive towards 300 microM of the alpha6-specific inhibitor furosemide (82% of control response). The sensitivity increased in older animals at the EC5-20 of GABA (31% of control responses), supporting an increased expression of alpha6-subunits as molecular basis for the observed developmental changes. Approximately 50% of cells in the age range P15-P48 were potentiated by 1 microM diazepam and by 3 microM methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM), suggesting the concurrent presence of alpha1- and alpha6-containing receptors, whereas the remaining of cells were neither potentiated by diazepam nor did they show the alpha6-typical DMCM potentiation, though they were potentiated by loreclezole. These properties indicate unknown pharmacological characteristics of cerebellar receptor-subunit combinations in approximately 50% of granule cells in situ.
Hevers
W
W
Department of Psychiatry, Clinical Research Group, University of Mainz, Untere Zahlbacher Strasse 8, 55131, Mainz, Germany. hevers@mail.uni-mainz.de
Lüddens
H
H
D000375:Aging / Q000502:physiology; D000465:Algorithms; D000818:Animals; D001640:Bicuculline / Q000494:pharmacology; D002243:Carbolines / Q000494:pharmacology; D002531:Cerebellum / Q000166:cytology* / Q000187:drug effects / Q000254:growth & development*; D003292:Convulsants / Q000494:pharmacology; D003975:Diazepam / Q000494:pharmacology; D004232:Diuretics / Q000494:pharmacology; D004594:Electrophysiology; D005665:Furosemide / Q000494:pharmacology; D018756:GABA Antagonists / Q000494:pharmacology; D018757:GABA Modulators / Q000494:pharmacology; D066298:In Vitro Techniques; D008564:Membrane Potentials / Q000187:drug effects; D018408:Patch-Clamp Techniques; D051381:Rats; D017207:Rats, Sprague-Dawley; D011963:Receptors, GABA-A / Q000187:drug effects*
D002243:Carbolines; D003292:Convulsants; D004232:Diuretics; D018756:GABA Antagonists; D018757:GABA Modulators; D011963:Receptors, GABA-A; C034818:methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate; D005665:Furosemide; D003975:Diazepam; D001640:Bicuculline