Hemispheric asymmetry of synapses in chick medial hyperstriatum ventrale following passive avoidance training: a stereological investigation

Abstract

A stereological analysis was made of synapses in the left and right hemispheres of chick medial hyperstriatum ventrale (MHV) 24h after passive avoidance training (PAL) and in water trained controls (W-control). The synaptic parameters examined were (D̄), the mean length of the postsynaptic thickening; (N V.syn), the number of synapses per unit volume of neuropil; (V V.syn), the volume density of the pre-synaptic bouton; (N V.ves), the number of synaptic vesicles per unit volume of neuropil and ( ves .syn), the mean number of synaptic vesicles per pre-synaptic bouton. No significant differences exist in N V.syn between the left or right hemispheres of W-control and trained chicks, nor is N V.syn influenced by training. However, in W-control chicks D̄ in the right MHV is significantly greater (12%) than in the left MHV and this difference disappears on training. There are no differences in V V.syn between left and right hemispheres of W-control chicks but following training V V.syn is 22.7% greater in the left MHV than in the right MHV. Training als influences the number of synaptic vesicles; in W-control chicks N V.ves in the right MHV is 12.25% greater than in the left MHV but following training these differences are reversed. When the data are expressed as numbers of vesicles per synapse ( ves .syn), values for the left hemisphere of trained chicks exceed those in the right hemisphere by a staggering 61.38%. These results are discussed in the context of biochemical and electrophysiological studies which suggest that there is lateralization of the memory trace.