The influence of ethanol intoxication on cholinergic axon terminals in the rat brain. (Morphometric evaluation)

In view of the inhibitory influence of ethanol on synaptic transmission demonstrated by numerous authors, the present investigations were undertaken with the purpose of ascertaining the ultrastructural changes in the cholinergic axon terminals of the CNS caused by chronic ethanol intoxication. The experiments were made with 12 rats receiving ethanol according to RATCLIFFE'S model (1972) in increasing doses over a period of 1, 2 and 3 months. The studies comprised evaluation of the electronmicroscopic picture of synapses from the temporal cortex and morphometric analysis of the number of synaptic vesicles in the boutons, of the surface area of synapse cross sections and the surface area of the cross sections of mitochondria in the boutons. Morphometric analysis showed that chronic administration of ethanol to rats reduces the number of synaptic vesicles in the boutons, which may be caused by chronic excitation of neurons by ethanol. Impairment of mitochondria within the synaptic boutons was also noted, probably due to disturbances in cell respiration produced by ethanol. Polymorphism of synaptic vesicles was observed in the form of greatly enlarged ones. It may be interpreted as the morphological manifestation of the process of axon terminals degeneration. Moreover, planimetric analysis demonstrated a significantly increased surface area of synaptic bouton cross sections in all experimental rats as compared with the control ones. The dynamics of the observed changes was not uniform. The reduction of the number of synaptic vesicles and impairment of mitochondria were most pronounced in the early phase of intoxication. In the later period in spite of increased doses and prolonged time of ethanol treatment the changes became rather less intensive, this probably being connected with the process of adaptation. Polymorphism of the synaptic vesicles became more pronounced gradually in the successive experimental groups, probably with corresponding advancing degeneration of the axon terminals. The significantly enlarged surface area of the synaptic boutons cross sections did not show any dependence on the connection of ethanol and duration of its administration.