The role of γ-aminobutyric acid as a mediator of positive dorsal root potentials

Abstract

Summary (1) A diphasic response consisting of a negative-, followed by a positive-going dorsal root potential (DRP) was evoked in L6 or L7 dorsal rootlets of unanesthetized spinal cats by 10 times group I threshold stimulation of the gastrocnemius-soleus muscle nerve. (2) The administration of 2.5 mg/kg bicuculine reversibly abolished, within 30 sec, first the negative and then the positive DRP. The time course for the recovery of the negative DRP was longer than that of the positive DRP. (3) Smaller doses of bicuculline (0.2–1.0 mg/kg) were usually much more effective against the negative than the positive DRP in terms of both magnitude and duration of suppression. (4) A selective action against the negative DRP was also observed, but with less consistency, after picrotoxin (3.0 mg/kg) administration. (5) Semicarbazide (200 mg/kg) induced a gradual diminution and often abolition of both DRPs over the course of 3 h. A selective reduction of the negative DRP occured after 60 min. (6) Suppression of the positive DRP by agents which block the receptor attachment (bicuculline and picrotoxin) and synthesis (semicarbazide) of GABA suggests that GABA is a transmitter in the pathway(s) mediating primary afferent hyperpolarization. (7) The possibility that GABA synapses occur in each of two separate pathways leading to the afferent terminal is discussed. According to this model, the positive DRP represents the GABA-mediated inhibition of a tonic non-GABA-mediated PAD; the negative DRP is conducted through a separate pathway terminating in a GABA-mediated axo-axonic synapse. The differential susceptibility would reflect, according to this model, anatomical or physiological differences between the GABA synapses of the respective pathways.