ANALYSIS OF SOME EVOKED SYNAPTIC ACTIVITIES 441 



which s.c.r.'s are compounded of inhibitory and excitatory synaptic activities 

 at various developmental stages, and (2) the distribution of components of 

 inhibitory p.s.p.'s in evoked s.c.r.'s recorded at different loci from the site of 

 stimulation. Information is also provided on the effects of aliphatic a>-amino 

 carboxylic acids that permits further analysis of their structure-activity 

 relations (Purpura et al., 1959b). 



Weak stimulation of the neocortical surface in neonatal kittens evokes a 

 complex superficial negativity whose duration is presumably dependent on 

 temporal dispersion of "unit" 10-20 msec p.s.p.'s generated in apical dendrites 

 of pyramidal neurons and dendrites of Cajal-Retzius cells. Of considerable 

 interest is the finding that dilute solutions of GABA exert a predominant 

 blocking action on late components of the s.c.r. Since the blockade of super- 

 ficial negativity depends on achieving an effective concentration of the 

 amino acid at synaptic sites responsible for the s.c.r., the relatively rapid 

 ehmination of late components suggests that the latter arise in dendritic 

 elements lying entirely within the molecular layer, whereas the initial 10-20 

 msec response is generated at postsynaptic loci along portions of apical 

 dendritic shafts subtending the outermost layers of cortex (Purpura and 

 Grundfest, 1956; Purpura et al., 1960b). Thus, although all components of 

 the s.c.r. are inferred to be p.s.p.'s, it is not unlikely that the second and 

 successive 10-20 msec negativities characteristically seen in near and distant 

 s.c.r.'s evoked in immature, as well as mature cortex (Brooks and Enger, 

 1959; Fan and Feng, 1957) arise in different dendritic organizations than those 

 responsible for the initial component. The major developmental changes in 

 these organizations, as revealed by the effects of Ce and Cg, provide addi- 

 tional support for this hypothesis. In the neonatal period, both long chain 

 oj-amino acids depress all components of the s.c.r., an action which tends to 

 resemble the effects observed on s.c.r.'s evoked in the cerebellar cortex of 

 adult cats (Purpura et al., 1959a, b). At a later stage of development (second 

 postnatal week) Cg augments late components of the near s.c.r. and, to a 

 lesser extent, Ce acts similarly. Not until the third and fourth week is it 

 possible to observe effects of Ce and Cg on near s.c.r.'s that are entirely 

 similar to those observed in the mature animal. If, as has been proposed 

 elsewhere (Purpura et al., 1959b), the lack of Ce and Cg action on an evoked 

 response indicates the absence of a component of inhibitory axodendritic 

 activity in a tested response, the changes in the pharmacological effects of 

 long chain w-amino acids may be considered reflections of the progressive 

 functional development of inhibitory activity in superficial axodendritic organ- 

 izations; first in organizations involved in the production of late components 

 in the s.c.r., then in those responsible for the early. An alternative explanation 

 of the different actions of long chain c^t-amino acids on neocortical s.c.r.'s 

 during postnatal ontogenesis based on the possibility that such differences 

 may result from changes in the pharmacological properties of inhibitory 



