Effects of proline and other neutral amino acids on ventral root potentials of the frog spinal cord in vitro.

Responses of frog motoneurones in vitro to bath-applied amino acids were studied by means of extracellular recordings from ventral roots in the presence of tetrodotoxin. Proline, GABA or taurine produced Cl^?-dependent depolarizations or hyperpolarizations of the ventral roots; these effects were analysed in a quantitative manner and fitted by dose-response relationships. Proline was a hyperpolarizing agent more potent than GABA or taurine. The different polarity of the amino acid responses might have been due to changes in the Cl^? equilibrium potential. However, hyperpolarizations following high doses (>2 mM) of proline were probably mediated by a different mechanism (perhaps increased K⁺ permeability) as suggested by the different kinetics and ion-sensitivity of this phenomenon. Hyperpolarizations induced by proline were antagonized by strychnine but not by picrotoxin (the opposite was found in the case of GABA) while those due to taurine were reduced by either convulsant. Amino acid-evoked depolarizations were almost insensitive to convulsants. It is concluded that proline in low concentrations is a fairly potent amino acid and that it might have a physiological role in certain synaptic mechanisms at spinal level.