Short-term changes in the numerical density of synapses in the intermediate and medial hyperstriatum ventrale following one-trial passive avoidance training in the chick

Abstract

Previous ultrastructural studies using stereological counting techniques, based on assumptions regarding shape, size, and orientation of synapses, have suggested synaptic remodeling occurred at least 24 hr after one-trial passive avoidance training in day-old chicks. The present study estimates the mean synaptic density (Nv syn) in a region of the chick forebrain known to be involved in memory formation, the intermediate and medial hyperstriatum ventrale (IMHV), 1 and 24 hr following one-trial passive avoidance training. A stereological technique, the “disector,” that makes no assumptions about size, shape, and orientation of synapses was used in the synaptic analyses. The density of axospinous synapses increased by approximately 77% at 1 hr posttraining in the right IMHV of chicks (M-trained) that learned to avoid a bitter-tasting bead, compared to those (W-trained controls) that peck a water-coated bead. A measure of the postsynaptic density size, the mean projected height of synapses (H), was 57% smaller 1 hr posttraining in the right IMHV of M-trained chicks. These differences were not found at 24 hr posttraining. We suggest that structural modification of synapses may be a key part of the processes involved in short-term memory formation.

Light Color Changes The Density and Height of Synapses in an Area of the Brain Essential For Learning

“…Studies, which have tested the visual discrimination in the chick, have consistently shown the importance of the right eye and the left hemisphere pathway in acquisition of such tasks (Rogers, 1986). The results presented here in which the left hemisphere has a superiority to the right hemisphere in terms of increase in N V syn after training in certain illumination colors are consistent with the idea of the importance of the left hemisphere in visual discrimination learning (Gasten and Gasten, 1984;Rogers, 1986;Patel and Stewart, 1988;Doubell and Stewart, 1993).…”

“…Clearly an increase or a decrease in N V syn may not reflect an increase or a decrease in number of synapses if the reference volume decreases or increases by a relevant amount. It was reported that after 24 hr passive avoidance training no changes occurred in the volume of the lobus parolfactorius (LPO), a region of chick forebrain Hunter (1991) cited by Doubell and Stewart (1993). One day after training, a similar result can be expected for the IMHV (Doubell and Stewart, 1993).…”

“…In other words, there was no dimorphism in IMHV between sexes, which were exposed to the same light color. In many of the stereological studies dealing with synapse density and synapse height in the chick IMHV were used both sexes and their comparisons were between the left and right side of IMHVs but not between sexes (Bradley et al, 1994;Doubell and Stewart, 1993;Stewart et al, 1984;Mayhew and Gundersen, 1996). Therefore N V syn values of the corresponding hemispheres of male and female chicks were summed, and the average mean of N V syn was taken for both sexes.…”

“…Many of the studies in which the stereological methods were applied both male and female chicks were used but data were not given separately for sexes (Stewart et al, 1984) or reports did not mention about sexes (Stewart et al, 1991;Doubell and Stewart, 1993;Bradley et al, 1994). To know whether there are any significant differences between the IMHV of sexes, male and female chicks were used in the present study.…”

“…It has been suggested that pre and post hatching visual experience may change synaptic characteristics in the IMHV in some way (Bradley et al, 1994). These studies advocate that an increase in synapse density (Hunter and Stewart, 1993;Doubell and Stewart, 1993), synaptic vesicles numbers (Stewart et al, 1984) and the post synaptic length can be expected in the young chicks after learning (Stewart et al, 1991).…”