ANALYSIS OF SOME EVOKED SYNAPTIC ACTIVITIES 443 



lation of lateral thalamic projections. Topical GABA eliminated the initial 

 surface-negativity and unmasked a surface-positivity whose amplitude was 

 dependent on stimulus strength (Fig. 15 (3), (4)). Following recovery from the 

 effects of GABA (9, 10), topical strychnine (0-1%) first reduced the initial 

 surface-negativity and activated a second, long-latency negativity which 

 progressively increased in ampHtude (11-14). During the height of the 

 strychnine action, GABA was reappHed and effects were produced that were 

 entirely similar to those observed before strychnine (15, 16). Upon removal 

 of GABA the initial negativity was barely detectable at first (17), then within 

 a few seconds strychnine effects were again observed (18). Strychnine depres- 

 sion of the early and appearance of late negativity in a surface response 

 evoked by thalamic stimulation in the new-born animal resembles the de- 

 pression of the early and augmentation of late components in the s.c.r. after 

 Ce and Cg in the 2 week-old-kitten. The effect of GABA (Fig. 15 (3)) is similar 

 to that produced in s.c.r. 's evoked by strong surface stimulation (Fig. 6 (6)). 

 Comparison of the effects of strychnine on a thalamocortical response and of 

 Ce and Cg on the s.c.r. illustrates that in the new-born animal inhibitory 

 activities may be prominent in some cortical organizations and not in others. 

 In the example shown in Fig. 15, the effects of strychnine are presumably 

 related to its blocking action at axosomatic (cf. Eccles, 1957), as well as 

 axodendritic inhibitory synapses (Purpura and Grundfest, 1957). However, 

 in view of the failure of GABA to reveal a surface-positivity other than 

 that which appears to reflect sub-surface depolarizing p.s.p.'s, it is likely 

 that the late negativity induced by strychnine results exclusively from blockade 

 of axosomatic inhibitory p.s.p.'s. The latter action may also explain the 

 appearance of modified intermittent paroxysmal discharges in the heavily 

 strychninized cortex of the new-born rabbit (Bishop, 1950; Fan, 1957) and 

 3-4 days old rat (Crain, 1952). Strychnine-induced depression of the initial 

 negative component of the thalamocortical-evoked response and Ce and Cg 

 depression of the s.c.r. prior to the second to third postnatal week indicates 

 that although the predominant blocking effect of these convulsants is exerted 

 on inhibitory synapses, depression of excitatory synaptic activity may also 

 occur (Purpura and Grundfest, 1957). 



The depressant effects of Ce and Cg on s.c.r.'s evoked in neonatal cortex 

 appear to be only partly accounted for by assuming that these compounds 

 have a minor affinity for excitatory axodendritic synapses. Ce and Cg mar- 

 kedly depress near and distant s.c.r.'s in neonatal cortex, but at a develop- 

 mental stage when these compounds cause rapid augmentation of near s.c.r.'s, 

 they produce no significant alteration in distant responses. Failure of Ce to 

 depress distant s.c.r.'s evoked in mature cortex may indicate that some altera- 

 tion has occurred during cortical maturation that tends to prevent long-chain 

 oj-amino acids from gaining access to excitatory axodendritic synapses. 

 Possibly some clues to the nature of this alteration may be forthcoming when 



