Evidence for cross-linking of cyclic AMP to constituents of brain tissue by aldehyde fixatives: potential utility in histochemical procedures.

The amount of cyclic AMP recovered from unfixed tissue sections of brain carried through immunohistochemical procedures was found to be small (less than 2 pmoles/mg protein) and constant despite wide variations in the amount accumulated prior to freezing and sectioning. However, treatment of brain slices with glutaraldehyde or formaldehyde in buffered sucrose solutions for one to two hours at 0 degrees rendered insoluble as much as 60% of the total accumulated cyclic AMP as judged by filtration of homogenates of treated tissue. Immunoreactive cyclic AMP was recovered from filters by extraction with warm, dilute acid. The fraction of filter-bound cyclic AMP was relatively constant over a wide range of initial tissue levels. The persistence of insoluble cyclic AMP was greatest in homogenates of slices treated with formaldehyde when maintained above pH 8.5; about 40% of this fraction was lost after two hours at 0 degrees. Incubation of tissue homogenates with formaldehyde and radioactive nucleoside or nucleotide derivatives of adenine, guanine, and cytosine resulted in a time-dependent appearance of insoluble radioactivity; compounds lacking an amino function were inactive. Pure proteins, including polylysine, also reacted with formaldehyde and 3H-cyclic AMP to produce radioactivity resistant to adsorption by charcoal. The reaction between cyclic AMP or adenosine, formaldehyde, and alkyl amines was examined using UV spectrometry. It is tentatively concluded that at 0 degrees formaldehyde is capable of rapidly producing a methylene-bridged adduct between primary alkylamines and the N6-amino function of purine nucleosides or nucleotides. Application of these results to the development of immunohistochemical procedures for the cellular localization of cyclic AMP will require the generation of antibody preparations with high reactivity for cyclic AMP derivatized at the N6-position.