POPULATION TRENDS IN THE FINE SPATIAL REORGANIZATION OF SYNAPTIC ELEMENTS IN FOREBRAIN REGIONS OF CHICKS 0.5 AND 24 HOURS AFTER PASSIVE-AVOIDANCE TRAINING

Two regions in the forebrain of domestic chicks (Gallus domesticus), t he intermediate and medial hyperstriatum ventrale and the lobus parolf actorius, have previously been shown to be important centres of bioche mical, pharmacological and physiological change following one-trial pa ssive avoidance training. The purpose of the present study was to exam ine, at the electron microscopic level, the fine spatial re-arrangemen t of synaptic structures in the intermediate and medial hyperstriatum ventrale (at 30 min), and in the lobus parolfactorius (at 24 h), post- training using comprehensive biometrical designs, image analysis and s tochastic approaches. In intermediate and medial hyperstriatum ventral e, no significant differences in the numerical density of synapses eit her between control and trained chicks, or between hemispheres, were r evealed using the disector method. However, after training, a nested-A NOVA demonstrated an increase in the thickness of pre- and post-synapt ic electron densities (estimated via image analysis) only in the left intermediate and medial hyperstriatum ventrale, whereas synaptic appos ition zone profiles increased in length bilaterally. In presynaptic te rminals from the intermediate and medial hyperstriatum ventrale, a sto chastic analysis revealed that training resulted in the re-distributio n of synaptic vesicles between two spatial pools relative to synaptic apposition zones, in both hemispheres producing a large number of syna ptic vesicles closer to synaptic apposition zones; a nearest neighbour analysis of synaptic apposition zone profiles indicated that the late ral shape of the synaptic apposition zone after training is more compl ex in both hemispheres. In the lobus parolfactorius at 24 h post-train ing the main changes in synaptic fine structure involved a shift of sy naptic vesicles away from synaptic apposition zones in the right hemis phere with the distance between synaptic apposition zones decreasing; in the left lobus parolfactorius, synaptic apposition zones became mor e regular/round in shape with a greater distance between them after tr aining. These data suggest that the initial acquisition of memory invo lves population changes in the fine spatial organization of synaptic v esicles and synaptic apposition zones in synapses in the intermediate and medial hyperstriatum ventrale, which indicate a possible tendency towards greater synaptic efficacies. These changes are as dynamic as t he molecular changes which have hitherto been considered the preserve of short-term correlates of memory formation.