GABA(B) receptor function in the ileum and urinary bladder of wildtype and GABA(B1) subunit null mice

1. GABA(B1) receptor subunit knockout mice were generated and the effects of the GABA(B) receptor agonist, baclofen, were evaluated within the peripheral nervous system (PNS) of wildtype (+/+), heterozygote (+/-) and knockout (-/-) animals. For this purpose, neuronally-mediated responses were evoked in both the isolated ileum and urinary bladder, using selective electrical field stimulation (EFS). 2. In ileum resected from 4-8-week-old-mice, low frequencies of EFS (0.5 Hz) evoked irregular muscle contractions which were prevented by atropine 1 microM and reduced by baclofen (33.4 +/- 5.6%, 100 microm). The latter effect was antagonized by the GABA(B) receptor antagonist CGP54626 0.2 microm. Baclofen 100 microm did not affect contractions of similar amplitude induced by carbachol, indicating that the ability of baclofen to inhibit cholinergic function in mouse ileum may be due to an action at prejunctional GABA(B) receptors. 3. To avoid the development of grand mal seizure by GABA(B1) (-/-) mice, a behaviour observed when the mice were greater than 3 weeks old, it was necessary to study the effects of this knockout in 1-3-week-old-animals. However, at this age, EFS at 0.5 Hz did not evoke robust muscle contractions. Consequently we used EFS at 5 Hz, which did evoke cholinergically mediated contractions, found to be of similar amplitude in (+/+) and (+/-) mice, of both 1-3 weeks and 4-8 weeks of age. At this frequency of EFS, baclofen reduced the amplitude of the evoked contractions [n = 6 (+/+) and n = 5 (+/-), IC50 19.2 +/- 4.8 microm) and this effect was greatly reduced in the presence of CGP54626 0.2 microm. 4. In urinary bladder from 1-3-week-old-mice, using higher frequencies of EFS to evoke clear, nerve-mediated contractions (10 Hz), baclofen 10-300 microm concentration-dependently inhibited contractions in (+/+) mice (IC50 9.6 +/- 3.8 microm). This effect was inhibited by CGP54626 (0.2 microm, 46.2 +/- 13.6% inhibition, 300 microm baclofen n = 7) a concentration which, by itself, had no effect on the EFS-evoked contractions. 5. The effects of baclofen in both ileum and urinary bladder were absent in the GABA(B1) receptor subunit (-/-) mice; however, responses to EFS were unaffected in (-/-) when compared to the (+/+) mice. 6. Our data suggest that, as in the central nervous system (CNS), the GABA(B1) receptor subunit is an essential requirement for GABA(B) receptor function in the enteric and PNS. As such, these data do not provide a structural explanation for the existence of putative subtypes of GABA(B) receptor, suggested by studies such as those in which different rank-orders of GABA(B) agonist affinity have been reported in different tissues.