No abstract

Activation of GABAB receptors evokes hypothermia in wildtype (GABAB(1)+/+) but not in GABAB receptor knockout (GABAB(1)−/−) mice. The aim of the present study was to determine the hypothermic and behavioural effects of the putative GABAB receptor agonist γ‐hydroxybutyrate (GHB), and of the GABAA receptor agonist muscimol. In addition, basal body temperature was determined in GABAB(1)+/+, GABAB(1)+/− and GABAB(1)−/− mice. GABAB(1)−/− mice were generated by homologous recombination in embryonic stem cells. Correct gene targeting was assessed by Southern blotting, PCR and Western blotting. GABAB receptor‐binding sites were quantified with radioligand binding. Measurement of body temperature was done using subcutaneous temperature‐sensitive chips, and behavioural changes after drug administration were scored according to a semiquantitative scale. GABAB(1)−/− mice had a short lifespan, probably caused by generalised seizure activity. No histopathological or blood chemistry changes were seen, but the expression of GABAB(2) receptor protein was below the detection limit in brains from GABAB(1)−/− mice, in the absence of changes in mRNA levels. GABAB receptor‐binding sites were absent in brain membranes from GABAB(1)−/− mice. GABAB(1)−/− mice were hypothermic by approximately 1°C compared to GABAB(1)+/+ and GABAB(1)+/− mice. Injection of baclofen (9.6 mg kg−1) produced a large reduction in body temperature and behavioural effects in GABAB(1)+/+ and in GABAB(1)+/− mice, but GABAB(1)−/− mice were unaffected. The same pattern was seen after administration of GHB (400 mg kg−1). The GABAA receptor agonist muscimol (2 mg kg−1), on the other hand, produced a more pronounced hypothermia in GABAB(1)−/−mice. In GABAB(1)+/+ and GABAB(1)+/− mice, muscimol induced sedation and reduced locomotor activity. However, when given to GABAB(1)−/− mice, muscimol triggered periods of intense jumping and wild running. It is concluded that hypothermia should be added to the characteristics of the GABAB(1)−/−phenotype. Using this model, GHB was shown to be a selective GABAB receptor agonist. In addition, GABAB(1)−/− mice are hypersensitive to GABAA receptor stimulation, indicating that GABAB tone normally balances GABAA‐mediated effects. British Journal of Pharmacology (2003) 140, 315–322. doi:

No abstract

1 The effects of the novel GABA analogue (2R)-(3-amino-2-fluoropropyl)sulphinic acid (AFPSiA) on transient lower oesophageal sphincter relaxations (TLOSRs) were studied in the dog. In addition, the GABA A /GABA B selectivity was determined in vitro and in vivo, and the pharmacokinetics and the metabolism of the compound were studied in the dog and rat. 2 TLOSRs were reduced by 5578% after intragastric administration of AFPSiA at 14 mmol kg À1 and did not decrease further at higher doses. When evaluated 2 and 4 h after administration, the effect declined to 3776 and 1679%, respectively. Spontaneous swallowing was only significantly inhibited at 100 mmol kg À1 . 3 The oral availability of AFPSiA was 52717 and 7174% in the dog and rat, respectively. A fraction of AFPSiA was oxidised to the corresponding sulphonate, (2R)-(3-amino-2-fluoropropyl)-sulphonic acid (AFPSoA) after oral administration to the rat and dog. 4 In rat brain membranes, AFPSiA was found to have ten times higher affinity for rat brain GABA B (K i ¼ 4774.4 nM) compared to GABA A (K i ¼ 430746 nM) binding sites. The compound was a full agonist at human recombinant GABA B(1a,2) receptors (EC 50 ¼ 130710 nM). In contrast, the metabolite AFPSoA was considerably more selective for binding to rat brain GABA A (K i ¼ 3773.1 nM) vs GABA B (K i ¼ 68007280 nM) receptors. 5 In the mouse, high doses (1-8 mmol kg À1 ) of AFPSiA induced a rapid and mild hypothermia followed by a profound and sustained hypothermia at the higher doses tested (6 and 8 mmol kg À1 ). This effect was unaffected by the selective GABA B receptor antagonist CGP62349. AFPSoA (1 and 2 mmol kg À1 ) produced transient and moderate hypothermia while the hypothermic response was considerably larger at 4 mmol kg À1 . 6 It is concluded that AFPSiA inhibits but does not abolish TLOSRs in the dog. High doses of the compound induce hypothermia in the mouse, which probably is attributable to activation of the GABA A receptor. The latter effect may be caused both by AFPSiA and its oxidised sulphonic acid metabolite AFPSoA.

No abstract