Effect of collidine (2,4,6-trimethylpyridine) on the osmiophilia and chromaffin reaction in the synaptic vesicles of rat pineal nerves

Rat pineal nerve endings contain a population of small and of large synaptic vesicles that are either electron lucent or have electron-dense cores. It has been reported that their osmiophilia is eliminated when collidine buffer is used in the fixation procedure. We investigated this effect and found that osmium tetroxide and potassium dichromate reactivity were abolished when excised pineal glands were briefly incubated with collidine buffer before glutaraldehyde-cacodylate fixation. Such an effect was not observed when collidine was applied after fixation. Glands that had been fixed in glutaraldehyde or osmium tetroxide buffered with collidine exhibited a peripheral zone containing reactive synaptic vesicles and a deeper, central zone where such reactivity was absent. These results indicate that the effect of collidine is due to depletion of monoamines rather than to chemical blockage of their reactivity, and further suggest that collidine has a higher rate of penetration into tissues than the tested fixatives.