A Study of the Role of GABAA- and GABAB-Receptors in Presynaptic Inhibition of Primary Afferents in the Spinal Cord of the Frog Rana ridibunda

The role of GABA^sub A^- and GABA^sub B^-receptors in presynaptic inhibition of primary afferent fibers was studied on an isolated preparation of the spinal cord of the frog Rana ridibunda. It is shown that the inhibitory effect of GABA on synaptic transmission from afferent fiber to motoneuron is caused by activation of both GABA^sub A^- and GABA^sub B^-receptors. A temporal correlation (± 5 min) was shown between the blocking action of bicuculline (a specific antagonist of GABA^sub A^-receptors) on primary afferent fiber depolarization (PAD) and its potentiating effect on the excitatory postsynaptic potential (EPSP) at parallel intracellular recording of EPSP in motoneuron and PAD in axons of the dorsal root. As a basis of this correlation, the single GABA^sub A^-receptor mechanism is discussed, which mediates the effect of bicuculline on PAD and EPSP. When a specific agonist of GABA^sub B^-receptor, baclofen, and an antagonist of GABA^sub B^-receptor, 2(OH)-saclofen, were applied, the obtained data indicated an involvement of GABA^sub B^-receptors in inhibition of synaptic transmission from afferent fibers to the motoneuron. Analysis of parameters of the unitary synaptic responses recorded in the control experiments and of their changes under the effect of (- )-baclofen indicates that the inhibitory action caused by activation of GABA^sub B^-receptors develops at the presynaptic level.[PUBLICATION ABSTRACT]