Hippocampal slices are electrically stimulated in the perforant path with a pulse-train, which can lead to long-term potentiation (LTP). Of the thus stimulated slices, subcellular fractions are prepared and used in an endogenous protein phosphorylation assay. A phosphoprotein band which was reported earlier to be sensitive to electric stimulation as well as to methionine-enkephalin is now further analyzed: it consists of two phosphoproteins only slightly differing in molecular weight: 50,000 Mr (50 K) and 52,000 Mr (52 K), but having distinct biochemical properties and subcellular localization. Their IEP is dissimilar (3.5-4.3 and 5.3, respectively), they display different sensitivity towards calcium when tested in the phosphorylation assay, but are both cAMP-independently phosphorylated. Only one of them responds to tetanic stimulation with an increased phosphorylation post hoc. This protein, the 52 K component, is localized in synaptic membranes. Moreover, this protein also responds to incubation of slices with methionine-enkephalin. The phosphorylation of the 50 K component is not influenced by electric stimulation, nor by incubations with neuropeptides; its phosphorylation takes place in material sedimenting with the mitochondrial cell fractions and is strongly calcium- and calmodulin-dependent.
Conduction velocities (c.vs) of different hippocampal fibre groups have been measured in slices in vitro. In the mossy fibres, Schaffer collaterals and Str. oriens fibres the c.v. varied from 0.36-0.38 m/s; in the alveus c.v. was 1.20 m/s. In the perforant path (p.p.) two groups of c.vs were found: in the medial p.p. the mean (+/- s.e) c.v. was 0.32 +/- 0.02 m/s, whereas in the lateral p.p. it was 1.36 +/- 0.14 m/s.
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