Pentylenetetrazol and reflex activity of isolated frog spinal cord

The superfused in vitro frog spinal cord preparation was used to investigate the effects of pentylenetetrazol (PTZ) on the spinal cord. PTZ depressed monosynaptic, but augmented polysynaptic reflexes, and decreased primary afferent deplorization. Concurrently, in Ringer's solution containing sufficient magnesium or cobalt ions to block synaptic transmission, PTZ antagonized the hyperpolarizing effects on motoneurons and the depolarizing effects on primary afferent fibers of the inhibitory amino acids GABA, beta-alanine, taurine, and glycine. PTZ did not affect responses to the excitatory amino acids glutamic acid and aspartic acid. Furthermore, PTZ did not alter high affinity uptake by cord slices, K+ -evoked release of [3H]GABA from them, or the spinal concentration of GABA. These data suggest that PTZ may produce its excitatory effects by postsynaptic blockade of inhibitory processes mediated by GABA (and possibly by other amino acids).