as normal faults in Fig. 3 are actually complex 26. R. B. Herrnann and J. A. Canas, Bull. Seismol. 30. See, for example, L. R. Sykes, Rev. Geophys. zones with multiple offsets. Some of the faults Soc. Am. 68, 1095 (1978); R. B. Herrmann, J. Space Phys. 16, 621 (1979). can be interpreted as high-angle reverse faults Geophys. Res. 84, 3543 (1979). 31. W. Stauder et al., Cent. Miss. Val. Earthquake (line S-6). 27. R. R. Heinrich, Bull. Seismol. Soc. Am. 31, 187 Bull. 19 (1979). 23. R. G. Stearns, U.S. Nucl. Regul. Comm. Rep. (1941). 32. We thank R. E. Anderson, W. H. Diment, and NUREGICR-0874 (1979). 28. T. G. Hildenbrand, personal communication. 0. W. Nuttli, who reviewed the manuscript and- 24. W. M. Caplan, Arkansas Div. Geol. Bull. 20 29. The research well near line S-10 encountered suggested improvements. Appreciation is ex- (1954). highly fractured and altered Paleozoic rocks. tended to the U.S. Nuclear Regulatory Commis- 25. T. C. Buschbach, U.S. Nucl. Regul. Comm. The mineralogy and geochemistry of the rocks sion for providing the funds for the research Rep. NUREGICR-0450 (1978). suggests a nearby igneous body. well. addition, analgesia that follows electrical stimulation of the brainstem of rats could be partially reversed by the opiate antag- onist naloxone, an indication that a mor- phine-like substance was being released Brain Peptides as Neurotransmitters (2). To identify the postulated morphine- like factor two approaches were taken. Solomon H. Snyder Hughes (3) showed that brain extracts can mimic morphine's effects on electri- cally induced contractions of smooth muscle in a fashion that is blocked by It is generally agreed that information represent only 10 percent or less of the naloxone. Terenius and Wahlstrom (4) processing in the brain largely involves total, whose number then may exceed and Pasternak et al. (5) identified in brain communication among neurons through 200. extracts a substance that competes for release of neurotransmitters at synapses. There has been much debate as to cri- opiate receptor binding. The specificity In theory, the brain might make do with teria that should be fulfilled before it can of this effect was established by showing one excitatory and one inhibitory trans- be designated a "'neurotransmitter." In that the marked regional variations in mitter. Until the 1960's the amines ace- this article, I regard as transmitters those opiate receptor density are paralleled by tylcholine, norepinephrine, and sero- peptides localized in specific neuronal similar variations in concentrations of the morphine-like substance (1, 3, 5). Naloxone blockade of the morphine-like Summary. Numerous peptides appear to be neurotransmitter candidates in the actions on smooth muscle and a regional brain. Some, such as the opioid peptide enkephalins, neurotensin, and substance P, distribution closely mimicking that of the were first isolated from the brain. Peptides, such as cholecystokinin and vasoactive opiate receptor ensured that the sub- intestinal polypeptide, were known as intestinal hormones and later recognized as stance under study was biologically rele- brain constituents. Certain hypothalamic-releasing hormones, pituitary peptides, and vant to opiate receptors. Such guaran- blood-derived peptides like angiotensin 11 and bradykinin, may also be central neuro- tees of biological relevance are impor- transmitters. The diversity of localization of these peptides throughout the brain im- tant, as attempts to isolate a substance plies a multiplicity of potential roles. solely based on its ability to inhibit the binding of a radioactive drug to mem- branes run the risk that the substance tonin were the only well-recognized systems and released on depolarization, being isolated may not be physiologically transmitters. Then came an appreciation which produce changes in neuronal ac- meaningful. that amino acids such as y-aminobutyric tivity, even though for the most recently Hughes et al. (6) isolated the mor- acid (GABA), glutamic acid, aspartic identified peptides some of these criteria phine-like substance from pig brain and acid, and glycine, might serve as trans- have not yet been examined. showed that it consists of two pen- mitters. A dramatic explosion in the tapeptides, methionine enkephalin (met- number of possible neurotransmitters enkephalin) and leucine enkephalin (leu- came with increasing recognition in the Opiate Receptor and Enkephalins enkephalin) which differ only in having past decade that various peptides may methionine or leucine at the carboxyl be neurotransmitters. At present there In most instances, neurotransmitters terminal. Using the assay based on com- seem to be about two dozen peptide neu- are identified as endogenous substances petition for receptor binding, Simantov rotransmitter candidates, and the num- and on this basis their receptor effects and Snyder (7) isolated the same two ber is increasing rapidly (Table 1). Most are characterized. In the case of the en- peptides from calf brain, confirming the of the brain peptide transmitters have kephalins, the receptors, which were dis- findings of Hughes et al. (6). Even before been discovered serendipitously with no covered first, provided a means to identi- the amino acid sequence of enkephalin systematic search. It would not be sur- fy and then isolate these opiate-like pep- was established, we showed by radio- prising if the known peptide transmitters tides. Dramatic properties of the opiate receptor assay that enkephalin was local- receptor, such as its discriminating agon- ized in nerve endings, which is consist- The author is Director of the Department of Neu- ists and antagonists and the intimate re- ent with a neurotransmitter role, that its roscience and Distinguished Service Professor of lation between receptor localization and detailed distribution in Neuroscience, Pharmacology and Experimental regional monkey Therapeutics, and Psychiatry and Behavioral Sci- central sites of pain perception (I) sug- brain closely paralleled that of opiate re- ences, Johns Hopkins University School of Medi- cine, 725 North Wolfe Street, Baltimore, Maryland gested that it might interact with a nor- ceptors, and that its phylogenetic distri- 21205. mally occurring opiate-like substance. In bution was the same as that of opiate re- 0036-8075/80/0829-0976$02.00/0 Copyright 1980 AAAS SCIENCE, VOL. 209, 29 AUGUST 1 2 www.sciencemag.org Downloaded from ceptors (8). Enkephalin is released in a Table 1. Peptide neurotransmitter candidates. have been identified, mu receptors, with calcium-dependent fashion with brain Gut-brain peptides distinct preference for morphine, and del- depolarization, further supporting a neu- Vasoactive intestinal polypeptide (VIP) ta receptors with selectivity for certain rotransmitter role (9). Cholecystokinin octapeptide (CCK-8) enkephalin derivatives, such as [D-Ala2- Once the amino acid sequence of en- Substance P D-Leu5Jenkephalin (Ala, alanine). Dif- Neurotensin kephalin was known it became evident Methionine enkephalin ferential autoradiographic localizations that the five amino acids (residues) con- Leucine enkephalin suggest that the two types of receptors stituting met-enkephalin are contained Insulin mediate different functions (20). Thus, within the 91 amino acids of the peptide Glucagon the mu receptors are preferentially local- /3-lipotropin isolated 10 years earlier Hypothalamic-releasing hormones ized to layers 1 and 4 of the cerebral cor- Thyrotropin-releasing hormone (TRH) from the pituitary by Li (/0). Several Luteinizing hormone-releasing tex, which (especially layer 4) are in- groups of investigators showed that a va- hormone (LHRH) volved preferentially in integrating sen- riety of lipotropin fragments, all incorpo- Somatostatin (growth hormone sory perception. The nucleus accumbens rating the met-enkephalin sequence, pos- release-inhibiting factor, SRIF) and olfactory tubercle, parts of the emo- Pituitary peptides sess opiate activity (11). Within the pitui- Adrenocorticotropin (ACTH) tion-regulating limbic system, are selec- tary most opiate-like activity can be ac- f3-Endorphin tively enriched in delta receptors. En- counted for by fl-endorphin, the 31 a-Melanocyte-stimulating hormone kephalin derivatives with preferential ac- amino acid peptide at the carboxyl termi- (a-MSH) tions at mu receptors are more potent an- nal portion of /3-lipotropin, although Others algesics, suggesting that mu receptors Angiotensin II another recently identified pituitary pep- Bradykinin mediate analgesic actions. Perhaps delta tide, dynorphin, also possesses high Vasopressin receptors preferentially regulate emo- opiate receptor activity (12). /8-Lipo- Oxytocin tional behavior. tropin itself derives from a 31,000-dalton Camosine My co-workers and I have proposed Bombesin precursor peptide which also incorporat- that the two types of opiate receptors in- es the sequence of adrenocorticotropic teract, respectively, with met- and leu- hormone (ACTH), and hence is referred enkephalins (20). Accordingly, the two to as 31K ACTH (13). /3-Endorphin oc- of sensory "pain" neurotransmitters types of enkephalin neurons may be in- curs in the brain at levels about 10 per- such as substance P. Enkephalin tracts volved, in part, in different brain func- cent those of enkephalin in specific neu- and opiate receptors in the limbic sys- tions. In support of this notion is the ronal systems (14) other than enkephalin tem, which regulates emotional behav- finding that brain regions with more mu neurons and has been reviewed