Efficacy of a putative GABA analog on synaptic transmission in the cat spinal cord.

4-Hydroxy-4-phenylcaproamide (HPhCA), at high doses or rates of IV injection depressed the ventral root reflexes elicited by nerve or dorsal root stimulation. The D (direct) and I (synaptic) ventral root waves and the antidromic (A) dorsal root wave evoked by intraspinal stimulation were also depressed. Similar effects were produced when HPhCA was applied topically on the cord dorsum. At 80 mg/kg and 8 mg/kg/min, the spinal reflexes and the I wave were facilitated for 4 to 6 h, but the D and A waves were depressed. Intracellular recordings from motoneurons showed that HPhCA injection produced: hyperpolarization that lasted several hours, short lasting (< 20 min) facilitation of both EPSPs and IPSPs as well as spike-like potentials (SLPs) that were triggered by EPSPs even though the neuron was hyperpolarized. SLPs may reach the threshold for full spikes. Our results suggest that the spinal depression results from hyperpolarization of motoneurons and the initial facilitation appears to be presynaptic. The late facilitation may be produced by SLPs. HPhCA does not appear to mimic the actions of GABA in primary afferents fibers and motoneurons.