A quantitative investigation of the development of collateral reinnervation after partial deafferentation of the septal nuclei

Abstract

A quantitative ultrastructural analysis of different types of synapses can yield figures which are highly consistent for the same area from one animal to the next. (1) In the medial and lateral septal nuclei of the rat, 64% of synapses are on dendritic spines, 32% are on dendritic shafts and 4% are on cell somata. (2) After section of the ipsilateral fimbria up to one half of the synapses on dendritic spines but only a small proportion of the synapses on dendritic shafts undergo orthograde degeneration. The degeneration reaches its peak within the first week and is virtually all removed by one month after operation. (3) The number of non-degenerating spine synapses falls to one half of its original value within a week after operation but returns to normal levels by one month. This suggests that almost all the deafferented dendritic spines may be reinnervated. (4) The number of axon terminals seen making more than one synaptic contact in the plane of section increases progressively from its normal low level to reach a maximum at about one month after operation. It is argued that these multiple synaptic contacts arise because of reinnervation of neighbouring vacated sites by pre-existing axon terminals in the region. In the formation of multiple synapses axon terminals already contacting dendritic spines are involved more often than can be explained on the basis of their proximity to the deafferented sites. (5) For the first few weeks after operation synaptic thickenings may be seen unapposed by axon terminals; at later stages these configurations become less common. This suggests that synaptic thickenings persist after deafferentation but that they exist for only a brief period in a non-synaptic form. Collateral reinnervation is not random — it is a quantitatively predictable phenomenon, following a rigid time course and resulting in a characteristic pattern of synapse formation.