GABA(A) receptor α subunits differentially contribute to diazepam tolerance after chronic treatment.
BACKGROUND
Within the GABA(A)-receptor field, two important questions are what molecular mechanisms underlie benzodiazepine tolerance, and whether tolerance can be ascribed to certain GABA(A)-receptor subtypes.


METHODS
We investigated tolerance to acute anxiolytic, hypothermic and sedative effects of diazepam in mice exposed for 28-days to non-selective/selective GABA(A)-receptor positive allosteric modulators: diazepam (non-selective), bretazenil (partial non-selective), zolpidem (α(1) selective) and TPA023 (α(2/3) selective). In-vivo binding studies with [(3)H]flumazenil confirmed compounds occupied CNS GABA(A) receptors.


RESULTS
Chronic diazepam treatment resulted in tolerance to diazepam's acute anxiolytic, hypothermic and sedative effects. In mice treated chronically with bretazenil, tolerance to diazepam's anxiolytic and hypothermic, but not sedative, effects was seen. Chronic zolpidem treatment resulted in tolerance to diazepam's hypothermic effect, but partial anxiolytic tolerance and no sedative tolerance. Chronic TPA023 treatment did not result in tolerance to diazepam's hypothermic, anxiolytic or sedative effects.


CONCLUSIONS
OUR DATA INDICATE THAT: (i) GABA(A)-α(2)/α(3) subtype selective drugs might not induce tolerance; (ii) in rodents quantitative and temporal variations in tolerance development occur dependent on the endpoint assessed, consistent with clinical experience with benzodiazepines (e.g., differential tolerance to antiepileptic and anxiolytic actions); (iii) tolerance to diazepam's sedative actions needs concomitant activation of GABA(A)-α(1)/GABA(A)-α(5) receptors. Regarding mechanism, in-situ hybridization studies indicated no gross changes in expression levels of GABA(A) α(1), α(2) or α(5) subunit mRNA in hippocampus or cortex. Since selective chronic activation of either GABA(A) α(2), or α(3) receptors does not engender tolerance development, subtype-selective GABA(A) drugs might constitute a promising class of novel drugs.
Division of Pharmacology, Utrecht Institute for Pharmaceutical Sciences, Utrecht University, Utrecht, The Netherlands. c.h.vinkers@uu.nl
Christiaan H
CH
Vinkers
Ruud
R
van Oorschot
Elsebet Ø
EØ
Nielsen
James M
JM
Cook
Henrik H
HH
Hansen
Lucianne
L
Groenink
Berend
B
Olivier
Naheed R
NR
Mirza
D000704:Analysis of Variance; D000818:Animals; D014151:Anti-Anxiety Agents / Q000008:administration & dosage / Q000494:pharmacology*; D001570:Benzodiazepinones / Q000378:metabolism / Q000494:pharmacology; D001831:Body Temperature; D001921:Brain / Q000378:metabolism*; D017931:DNA Primers / Q000235:genetics; D003975:Diazepam / Q000008:administration & dosage / Q000494:pharmacology*; D004361:Drug Tolerance / Q000235:genetics / Q000502:physiology*; D005442:Flumazenil / Q000378:metabolism; D006993:Hypnotics and Sedatives / Q000008:administration & dosage / Q000494:pharmacology*; D017403:In Situ Hybridization; D008297:Male; D051379:Mice; D008817:Mice, Mutant Strains; D011724:Pyridazines / Q000378:metabolism / Q000494:pharmacology; D011725:Pyridines / Q000378:metabolism / Q000494:pharmacology; D011963:Receptors, GABA-A / Q000378:metabolism*; D014230:Triazoles / Q000378:metabolism / Q000494:pharmacology; D000077334:Zolpidem
C507018:7-(1,1-dimethylethyl)-6-(2-ethyl-2H-1,2,4-triazol-3-ylmethoxy)-3-(2-fluorophenyl)-1,2,4-triazolo(4,3-b)pyridazine; D014151:Anti-Anxiety Agents; D001570:Benzodiazepinones; D017931:DNA Primers; D006993:Hypnotics and Sedatives; D011724:Pyridazines; D011725:Pyridines; D011963:Receptors, GABA-A; D014230:Triazoles; D005442:Flumazenil; D000077334:Zolpidem; C054626:bretazenil; D003975:Diazepam