Histamine H₃ receptors regulate the release of several neurotransmitters (Leurs et al. 2000). The anticonvulsant effects of thioperamide (H₃ antagonist/inverse agonist) are prevented by the H₃ agonist R-α-methylhistamine (R-α-MH) (Yokoyama et al. 1993). The GABA_A receptor is distributed throughout the CNS and is important in the pathophysiology and treatment of epilepsy. Native rat GABA_A receptors were antagonised by an H₃ agonist: here we examine recombinant human GABA_A receptors and the effects of the H₃ antagonist thioperamide.

Receptors were expressed in Xenopus oocytes by blind nuclear injection of GABA_A cDNAs (provided by Dr P. Whiting, MSD, UK). Two-electrode voltage clamp in amphibian Ringer, 1-5 days later, was used to measure GABA currents at 22-24 °C. Culture techniques and saline composition have been published (Verdon et al. 2000). Pyramidal cells were whole cell clamped at 22-24 °C. Salines for cultured cells contained 50 nM tetrodotoxin, 100 nM pyrilamine and 5 µM cimetidine. Data, presented as means ± S.E.M., were analysed (Prism 3 from Graphpad, Spike 2 from CED Ltd, and University of Strathclyde software) by paired, two-tailed Student’s t test or ANOVA as appropriate. P values of < 0.05 were significant.

At V_h of -70 mV, 100 nM R-α-MH reduced currents evoked by 10 µM pressure-applied GABA in cultured neurones (control, 1152.0 ± 182.8 pA, R-α-MH, 593.8 ± 93.8 pA; P = 0.0012, n = 4). Pretreatment of cultured neurones with 1 µM thioperamide significantly increased GABA currents, and blocked the depressant response to R-α-MH (control, 2938.0 ± 343.3 pA; thioperamide, 3063.0 ± 319.7 pA; P < 0.01; R-α-MH and thioperamide, 2769.0 ± 312.0 pA; P > 0.05, n = 4). Currents produced by 1 µM GABA in recombinant GABA_A receptors (α₁β₂γ_2L) were weakly antagonised by 100 nM R-α-MH, without concomitant expression of cloned H₃ receptors (control, 1377.0 ± 517.2 nA; R-α-MH, 1169.0 ± 511.1 nA; P = 0.045, n = 4). However, currents produced by a saturating concentration of GABA (100 µM) were not significantly altered by R-α-MH (P = 0.811, n = 4).

The H₃ receptor agonist R-α-MH has an antagonistic effect on GABA_A receptors in cultured neurones and in recombinant receptors in oocytes. Endogenous H₃ receptors have not been reported in oocytes. One interpretation of these data is that the ligands are not absolutely selective for metabotropic H₃ receptors but that they directly interact with CNS GABA_A receptors.

Thanks to The Wellcome Trust for equipment support. Histamine receptor ligands were kindly supplied by the James Black Foundation, King’s College London.