356 



H. MCLENNAN 



Fig. 5. Excitatory postsynaptic potentials recorded from a quadriceps motor 

 neuron in response to dorsal root stimulation. Upper record before, lower after 

 application of Factor I solution to the spinal cord. (McLennan, unpublished 



observations.) 



showed to have a potent blocking action on the crayfish stretch receptor 

 neuron. 



GABA was compared with Factor I in a number of situations where the 

 latter is active (McLennan, 1957; Florey and McLennan, 1959; Honour and 

 McLennan, 1960) and found not to have precisely similar effects. In particular 

 it had no action on monosynaptic reflexes even when appHed in high con- 

 centration to the spinal cord. The substance has been appfied to the cerebral 

 cortex by a number of workers with results which have been interpreted by 

 some as an indication of specific blocking of excitatory synaptic processes 

 (Grundfest, 1958). However, Curtis et al. (1959) have now provided con- 

 vincing evidence that the action of GABA is as a "depressor" on spinal 

 neurons, inasmuch as after local application to the vicinity of a cell it re- 

 duced both e.p.s.p.'s and i.p.s.p.'s without any alteration in the resting poten- 

 tial. Strychnine furthermore was without effect on these actions. It seems 

 apparent then that GABA cannot be the inhibitory transmitter of the mam- 

 mahan central nervous system. 



It is moreover possible to prepare active Factor I solutions which contain 

 little or no GABA (McLennan, 1958). There are a number of other related 

 compounds whose actions on the crayfish neuron (Edwards and Kufiler, 

 1959) and spinal neurons (Curtis et al., 1959) are similar to those of GABA. 

 Such are /S-alanine, S-aminovaleric acid, /3-guanidinopropionic and y-guani- 

 dinobutyric acids, y-amino-/3-hydroxybutyric acid, etc. All of these are pos- 

 sible constituents of the brain extracts: all, like GABA, fail to duplicate 

 entirely the actions of Factor I. McLennan (1960) has recently divided Factor 

 I into two active fractions whose biological actions are very similar but whose 



