Synaptic reorganization in the medial amygdaloid nucleus after lesion of the accessory olfactory bulb of adult rat. I. Quantitative and electron microscopic study of the recovery of synaptic density.

The time course of the appearance of degenerating synapses and the loss and reappearance of intact synapses was investigated in the medial amygdaloid nucleus (MAN) following lesions of the accessory olfactory bulb (AOB). Degenerating synapses were concentrated in the molecular layer of MAN. A quantitative electron microscopic analysis of the molecular layer was performed at 2, 4, 8, 16, 32 and 64 days after the placement of kainic acid injections within AOB. At each survival time, counts of synapses were made from photomicrographic montages. The degenerating synapses were observed at 2 days after lesion. Their density reached a peak at 2 or 4 days after the lesion and decreased thereafter. No degenerating synapses were found at 64 days following the lesion. Shrinkage of the thickness of molecular layer was observed following the AOB lesion. This was most pronounced at 4 or 8 days after the lesion at which time the thickness was 70-75% of the contralateral molecular layer. The shrinkage receded moderately (85% of contralateral layer at 64 days after lesion). The compensated synaptic density of intact synapses was reduced to less than half of the control density at 4 or 8 days after the lesion. Thereafter synaptic density increased gradually and reached more than 80% of control density at 64 days after lesion. The recovery in number of intact synapses following the lesion suggests the possibility of synaptic reorganization by remaining afferent fibers.