ANALYSIS OF SOME EVOKlED SYNAPTIC ACTIVITIES 433 



dissecting overt axodendritic synaptic activities into excitatory and iniiibitory 

 components derives from a large volume of data on the different effects of 

 these compounds on surface evoked responses in neocortex, hippocampus, 

 and cerebellar cortex of adult cats. These effects have been described in detail 

 elsewhere and consequently only features relevant to the present discussion 

 will be reviewed (Purpura, 1960a; Purpura et al., 1959a, b, 1960). 



Although a relatively large number of ahphatic cu-amino, co-guanidino and 

 oj-thioureido carboxylic acids produce rapid and reversible effects on neo- 

 cortical and cerebellar s.c.r.'s in adult animals, the different effects of these 

 compounds are represented by the action of two members of the cu-amino 

 acid series, the four carbon member, y-aminobutyric acid (GABA), and the 

 six carbon compound, e-amino caproic acid (Ce). Topically applied GABA 

 rapidly eliminates the neocortical and cerebellar s.c.r., but only in neocortex 

 does abolition of surface-negativity reveal a surface-positivity previously 

 "masked" in the overt response. In contrast, Ce augments neocortical s.c.r.'s 

 recorded 1-2 mm from the site of stimulation, whereas cerebellar s.c.r.'s are 

 relatively unaffected by this and other long-chain a»-amino acids. The effects 

 of GABA are ascribed to selective blockade of depolarizing axodendritic 

 p.s.p.'s, those of Ce to inactivation of hyperpolarizing, inhibitory axodendritic 

 p.s.p.'s (Purpura et al., 1959b). GABA-induced surface-positivity in the neo- 

 cortical s.c.r. is inferred to be a hyperpolarizing p.s.p. of superficial dendritic 

 elements. Its absence in cerebellar s.c.r.'s is thus attributed to the relative 

 paucity of inhibitory synapses on Purkinje cell dendritic terminals activated 

 by superficial fibers. The lack of effect of Ce and other long-chain tu-amino 

 acids on cerebellar s.c.r.'s evoked by local surface stimulation is in marked 

 contrast to the action of these compounds on cerebellar responses evoked by 

 stimulation of different afferent pathways (Purpura et al., 1959a). Of particular 

 importance in this respect is that the effects of long-chain oj-amino acids on 

 different varieties of evoked potentials are almost entirely reproduced by 

 topically applied strychnine. Analyses of the alterations in surface responses 

 induced by successive application of GABA and Ce or strychnine may there- 

 fore provide significant clues concerning the proportion, magnitude and 

 temporal relations of axodendritic and axosomatic p.s.p.'s activated by differ- 

 ent pathways (Purpura, 1960a). 



The typical effects of GABA on long-duration s.c.r.'s recorded 1-5 mm 

 from the site of stimulation in the new-born cat are shown in Fig. 6a. These 

 consisted in a rapid decrease in amplitude and duration of the surface- 

 negativity, and "unmasking" of a low-ampHtude, late surface-positivity 

 (Fig. 6a, (2), (3)). With continued GABA-action the low-amplitude surface- 

 positivity was further reduced (Fig. 7 (1-3)). During the early stages of 

 recovery from GABA-blockade, s.c.r. amplitude was greater than before 

 treatment with the amino acid, but late components of the response were 

 still depressed (Fig. 6a (4); Fig. 7 (4)). The phase of augmented excitability 



