Abstract
Antischizophrenic drugs, reduced in a concentration-dependent fashion enkephalin degradation by the soluble and particulate fractions of the human cerebral cortex and cerebellum. The order of potency is as follows: thioridazine > chlorpromazine > fluphenazine > haloperidol ≧ promazine with IC 50 of 50, 80, 120, 200–250 μM, respectively. Kinetic studies revealed non-competitive and competitive inhibition by thioridazine and chlorpromazine, respectively. Narcotics, were weak inhibitors of enkephalin degradation. For dl-, d-, l-methadone and l-α-acetylmethadol, IC 50 was about 500 μM, and 1000 μM for heroin and morphine. It is suggested that inhibition of the degradation of endogenous morphinomimetic peptides in the CNS may be a crucial factor governing the pharmacology of some neuroleptics and other psychoactive drugs. Enkephalin-hydrolyzing activity was ubiquitous and exhibited considerable regional differences in the normal human and in Huntington's chorea brains. The rate of enkephalin degradation is generally higher in the subcortical nuclei than in the cortex and cerebellum. The highest hydrolytic activity was found in the substantia nigra, anterior thalamus, septal area, globus pallidus and caudate nucleus, in this decreasing order.
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