Journal of Neuroscience, Vol 7, 291-301, Copyright © 1987 by Society for Neuroscience
Distribution of cAMP and cAMP-dependent protein kinases in Aplysia sensory neurons
SM Greenberg, L Bernier and JH Schwartz
Sensitization of the gill- and siphon-withdrawal reflex in Aplysia is
considered a simple form of learning. Previous work has provided
physiological and pharmacological evidence that cAMP-dependent protein
phosphorylation within identified sensory neurons of the abdominal ganglion
underlies the short-term form of this behavioral modification. Our main
goal in this paper is to determine the subcellular distribution of cAMP and
to measure the amounts and properties of the 2 types of subunits
(regulatory and catalytic) that constitute the cAMP- dependent protein
kinase. Do these biochemical parameters differ in sensory cells from those
in other parts of nervous tissue? We found that the increased cAMP
synthesized under conditions of sensitization is distributed in 3
compartments in the neuron: most of it is free in the cytoplasm; the
remainder is bound either to cytoplasmic or to particulate proteins, which
are believed to be regulatory subunits of the cAMP-dependent protein
kinase. Binding of cAMP within the neurons is a measure of activation of
the kinase. At rest, 17% of the binding sites in sensory cells were
occupied. After brief electrical stimulation of the connective, which
released endogenous transmitter, occupancy increased to 34%. This treatment
increased the amount of cAMP bound to the various binding proteins
differentially. The biochemical characteristics of cAMP binding were found
to be the same in sensory neurons as in the rest of the nervous system but
different from those in muscle. Thus, memory and learning are likely to be
mediated by enzymes that are shared by other nerve cells. We found that
sensory neurons have greater cAMP-dependent protein kinase activity than
other neurons, however, and as a result may be more sensitive to small
increases of cAMP.
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