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:

Transient receptor potential (TRP) channels play essential roles in sensory physiology and their expression in different classes of sensory neurons reflect distinct receptive properties of these neurons. While expression of the TRPV, TRPA, and to a certain degree TRPM classes of channels has been studied in sensory neurons, little is known about the expression and regulation of TRPC channels. In this study we examined the regulation of all TRPC members (TRPC1-C7) throughout embryonic and postnatal development of the dorsal root ganglion (DRG) and nodose ganglion (NG). In adult mice, mRNAs for all channels were present in the DRG, with TRPC1, 3, and 6 being the most abundant, TRPC2, C4, and C5 at lower levels, and TRPC7 at very low levels. While TRPC2 mRNAs were downregulated from high levels at embryonic (E) day 12 and E14 until adult, TRPC4, C5, and C7 expressions increased from E12 to peak levels at E18. TRPC1, C3, and C6, the most abundant TRPC channel mRNAs, increased progressively from E12 to adult. Expression and regulation of TRPC channels mRNAs in the NG were unexpectedly similar to the DRG. TRPC1 and C2 was expressed in the neurofilament-200 (NF-200)-positive large size subclass of neurons, while TRPC3 mRNAs expression, which stained up to 35% of DRG neurons, was almost exclusively present in nonpeptidergic isolectin B4 (IB4)-positive small size neurons that were largely TRPV1-negative. Our results suggest important roles of the TRPC family of channels in sensory physiology of both nociceptive as well as nonnociceptive classes of neurons.

The actions of the anticonvulsant gabapentin [1-(aminomethyl)-cyclohexaneacetic acid, Neurontin] have been somewhat enigmatic until recently, when it was claimed to be a ␥-aminobutyric acid-B (GABA B ) receptor agonist acting exclusively at a heterodimeric complex containing the GABA B(1a) splice variant (Mol Pharmacol 2001;59:144 -152). In this study, we have investigated the effects of gabapentin on recombinant GABA B(1a) and GABA B(1b) receptors coexpressed with GABA B(2) in five different functional recombinant assays, its ability to inhibit [ 3 H]GABA binding in a GABA B receptor-selective binding assay using rat synaptic membranes, and its ability to inhibit transient lower esophageal sphincter relaxations in Labrador retriever dogs. Up to a concentration of 1 mM, gabapentin displayed no agonistic effects on either the GABA B(1a,2) or the GABA B(1b,2) heterodimer, when these were expressed in Xenopus laevis oocytes or mammalian cells and assayed by means of electrophysiology, calcium mobilization, inositol phosphate, and fluorometry assays. Gabapentin did not displace [3 H]GABA from GABA B receptor sites in rat synaptic membranes. Finally, in contrast to the classic GABA B receptor agonist baclofen, gabapentin was unable to inhibit transient lower esophageal sphincter relaxations in dogs. Because of high levels of GABA B(1a) in the canine nodose ganglion, this finding indirectly supports the inactivity of gabapentin on the GABA B(1a,2) heterodimer demonstrated in various in vitro assays. In light of these results, we find it highly questionable that gabapentin is a GABA B receptor agonist. Hence, the anticonvulsive effects of the compound have to arise from GABA B receptor-independent mechanisms. This also implies that the first GABA B receptor splice variantselective ligand remains to be discovered.␥-Aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the central nervous system, where it exerts its effect through the ionotropic GABA A receptors and the metabotropic GABA B receptors. The GABA B receptors belong to the family C of the G-protein-coupled receptor superfamily (Möhler and Fritschy, 1999;Marshall et al., 2000). Two receptors, GABA B(1) and GABA B(2) , have recently been cloned, and several splice variants of both receptors have been iden-