Presynaptic GABAB receptors and the regulation of [3H]noradrenaline release from rat anococcygeus muscle.

The release of preaccumulated [3H]noradrenaline from the isolated rat anococcygeus muscle, a tissue endowed with a rich noradrenergic innervation, was investigated. Field stimulation produced a marked, calcium-dependent enhancement in release, which could be substantially inhibited by the inclusion of low concentrations of GABA or (-)-baclofen (but not the (+)-isomer) in the bathing medium. Classical GABAA site agonists such as muscimol or 3-aminopropanesulphonic acid were inactive, or only weakly so. Picrotoxin and bicuculline were unable to antagonise the effects of GABA or baclofen on [3H]noradrenaline release, although 5-aminovalerate was effective. These data provide further evidence for the existence of GABAB receptors associated with presynaptic noradrenergic terminals in the rat anococcygeus muscle.