The thiocholine technique for cholinesterase has been successfully adapted for the demonstration of enzyme distribution with the electron microscope. After fixation in buffered glutaraldehyde, thin slices of tissue were taken through a histochemical procedure designed to minimize diffusion artifacts and cytological damage; appropriate areas were dissected out, fixed with osmium tetroxide and embedded in Araldite.

This technique has been used to study the distribution of enzyme in and around known cholinergic neurons in the rat. In diaphragm muscle the intense staining of the motor end plates is confined to the actual synaptic clefts. The distribution of cytoplasmic staining was similar in all three types of cholinergic neurons studied (ventral horn cells from the cervical cord, cells from the dorsal motor nucleus of the vagus, and cells from the hypoglossal nucleus). There was intense staining of the space contained within the individual bilaminar sheets of the rough endoplasmic reticulum and occasionally in areas of the nuclear envelope. Mitochondria, lysosomes, the smooth endoplasmic reticulum and most of the plasma membrane were unstained.

In cholinergic nerve fibres staining was particularly intense at the axonal membrane and absent from the myelin sheath. In several regions of the brain known to receive an afferent cholinergic innervation many of the identifiable synaptic areas were heavily stained. Staining here was usually present round most of the presynaptic process, spreading over part of the postsynaptic process and often penetrating into the actual synaptic space. The mitochondria, microvesicles and the general cytoplasm of the synaptic processes were all unstained. In some areas, such as the hippocampus, there was intense membrane staining of fine cholinergic neuropil.

The enzyme specificity of the technique is not in doubt and evidence is adduced for the view that diffusion artifacts are small. The close correlation between the electron-microscopic results and evidence from physiological and biochemical investigations is discussed.

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