ANALYSIS OF SOME EVOKED SYNAPTIC ACTIVITIES 435 



after, effects similar to those previously demonstrated in adult animals were 

 readily obtained (Purpura et al., 1959b). In some instances when especially 

 strong surface stimulation was employed to evoke s.c.r.'s that were succeeded 

 by a prominent surface-positivity, i.e. the "deep response" of Adrian (1936), 

 GABA unmasked a short-latency surface-positivity of similar time course to 

 the pre-treated s.c.r. (Fig. 6b). Unlike the low-amplitude surface-positivities 

 commonly observed after GABA, those "unmasked" in s.c.r.'s evoked by 

 excessively strong stimulation were not abolished by continued application 

 of the amino acid. It is not unlikely, therefore, that whereas the low-ampli- 

 tude, late surface-positivity may be a sign of weak inhibitory p.s.p.'s in the 

 s.c.r., that revealed by strong stimulation probably reflects depolarizing 

 p.s.p.'s of sub-surface elements (Purpura et al., 1960b). 



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Fig. 8. Effects of topical GABA on early and late components of s.c.r.; 3-day-old 

 kitten. S.c.r.'s recorded 1-5 mm from stimulating electrodes. (1) Responses to 

 stimuli of progressively increasing strength, duration of s.c.r. evoked by weak 

 stimulus is identical to that evoked by strong. (2) Effects of topical GABA 

 (0 1%) on s.c.r.'s evoked by stimulus of constant strength (frequency 01/sec). 

 Response of greatest amplitude and duration is before GABA. The immediate 

 effect of the amino acid consists in a minimal uniform reduction in amplitude 

 of all components of the s.c.r. (second record from above-downwards). Later 

 effect is exerted primarily on late components which were eliminated by the 

 amino acid at a time when the initial 15 msec component was still prominent. 

 (3) (a) and (b) s.c.r.'s evoked by strong and weak stimulus, respectively; (c) 

 maximum effect of 001% GABA on response evoked by strong stimulus as 

 in (a). Further explanation in text. Cal. 100 c/s; 0- 1 mV. 



The disproportionate loss in late components of the s.c.r. following treat- 

 ment with GABA is of interest with respect to the hypothesis that the long- 

 duration of s.c.r.'s in immature cortex may be due to summated p.s.p.'s in 

 diff'erently oriented elements (Purpura et al., 1960a). This interpretation is 

 supported by observations on the effects of dilute GABA on early and late 

 components of the s.c.r. (Fig. 8). As noted previously (Fig. 2), the duration 

 of s.c.r.'s recorded at distances greater than 1-2 mm in neonatal kittens is 

 independent of stimulus strength (Fig. 8 (1)). After application of weak 

 GABA solutions (0-1 %,), loss of late components occurred faster than that 

 of the early (Fig. 8 (2)). When very weak GABA solutions (0-01 %) were 

 employed, depression of the s.c.r. proceeded to a stage at which the peak- 

 amphtude of the control response (labeled a in Fig. 8 (3)) had been reduced 

 by about 40% (c). The latter response (c), in contrast to the control response 

 of similar peak-amphtude that was evoked by a weak stimulus (b) was almost 



