Alterations in synaptic structure in the paleostriatal complex of the domestic chick, Gallus domesticus, following passive avoidance training.

A morphometric study was made of synapses in both left and right hemispheres of two regions of the chick paleostriatal complex, the paleostriatum augmentatum (PA) and the lobus parolfactorius (LPO), 24 h after passive avoidance training (methyl anthranilate, M-chicks), and in water-trained controls (W-chicks). The synaptic features examined were D, the mean length of the postsynaptic thickening; Nv.syn, the numerical density of synapses; Vv.syn, the volume density of the presynaptic bouton; V, the mean volume of the presynaptic bouton; Nv.ves, the numerical density of synaptic vesicles per bouton volume; ves.syn, the number of synaptic vesicles per presynaptic bouton; and K, the curvature of the synaptic contact zone. In the LPO there is a significant increase in the numerical density of synapses (Nv.syn) in both left and right hemispheres of M-compared with W-chicks (up to 59%, depending on the method of calculation used). A hemispheric asymmetry of postsynaptic thickening length (D) which is present in W-chicks (R greater than L by 10%) is reversed in M-chicks. There is no difference in the volume density of the presynaptic bouton (Vv.syn) or the mean bouton volume (V) either between W- and M-chicks, or between left and right hemispheres. Significant changes are found after avoidance training in both of the synaptic vesicle parameters measured. There is an increase of approximately 50%, both in the numerical density of synaptic vesicles (Nv.ves) and the number of vesicles per synaptic bouton (ves.syn), in the left hemisphere of M-chicks. No changes in the mean synaptic contact curvature (K) were observed after training, either of presynaptically concave, or presynaptically convex synapses, in either left or right hemispheres, nor did the percentage distribution of these different curvature classes vary greatly. In the PA there were no significant changes in D, Nv.syn or Vv.syn, either between M- and W-chicks, or between left and right hemispheres, 24 h after passive avoidance training. However, when the differences between the mean bouton volume (V) are examined, there is a significant increase in size of the boutons in the left hemisphere of M- compared to W-chicks. There is also a hemispheric asymmetry in both Nv.ves and ves.syn in W-chicks (R greater than L by approximately 15%) and this disappears on M-training. No changes in the mean synaptic contact curvature were observed after training, either of presynaptically concave, or presynaptically convex synapses, in either left or right hemispheres. However, the percentage distribution of these different curvature classes showed some variation.(ABSTRACT TRUNCATED AT 400 WORDS)