- Electrical stimulation in vivo increases the expression of proenkephalin mRNA and decreases the expression of prodynorphin mRNA in rat hippocampal granule cells. Morris, B.J., Feasey, K.J., ten Bruggencate, G., Herz, A., Höllt, V. Proc. Natl. Acad. Sci. U.S.A. (1988)