R. J. Miller, D. J. Reis, Soc. Neurosci. Abstr. 6,353 (1980); N. Aronin, M. DiFiglia, A. Liotta, J.B. Martin, ibid., p. 353; E. J. Glazer and A. I.Basbaum, ibid., p. 523.
10. E. Carstens and D. L. Trevino, J. Comp.Neurol. 182, 151 (1978); G. J. Giesler, Jr., D.Menbtrey, A. I. Basbaum, ibid. 184, 107 (1979);W. D. Willis, D. R. Kenshalo, Jr., R. B. Leon-ard, ibid. 188, 543 (1979); S. Hockfield and S.Gobel, Brain Res. 139, 333 (1978); H. Burtonand A. D. Craig, Jr., ibid. 161, 515 (1979).
11. D. L. Trevino, R. A. Maunz, R. N. Bryan, W.D. Willis, Exp. Neurol. 34, 64 (1972); W. D.Willis, D. L. Trevino, J. D. Coulter, R. A.Maunz, J. Neurophysiol. 37, 358 (1974); D. D.Price, R. Dubner, J. W. Hu, ibid. 39, 936 (1976);D. D. Price, R. L. Hayes, M. Ruda, R. Dubner,ibid. 41, 933 (1978); D. R. Kenshalo, Jr., R. B.Leonard, J. M. Chung, W. D. Willis, ibid. 42,1370 (1979); D. D. Price, H. Hayashi, R.Dubner, M. A. Ruda, ibid., p. 1590; G. J.
Comparisons of banded metaphasechromosomes (320 to 500 bands per hap-loid set) of man, chimpanzee, gorilla,and orangutan have revealed a generalhomology of chromosomal bands in thefour species and suggested a commonancestor for chimpanzee, gorilla, andman (1, 2). Using high-resolution G-banded chromosomes from late pro-phase (1000 bands per haploid set) (3),we can now account for every nonhet-erochromatic G-positive and G-negativeband in the four primates. Furthermore,by comparing chromosomes of humans,apes, and some Old World monkeys, wehave been able to work backward inevolution to suggest likely karyotypesfor three presumed common ancestors ofapes and man. This study was based onthe remarkable similarity of chromo-somes of man, chimpanzee, gorilla, andorangutan, the few changes needed toexplain their differences, and the use ofancestral chromosomal patterns to de-rive the general sequence of events thatmight have taken place in primate evolu-tion prior to man's emergence. Such anapproach suggests (i) the existence of aprecursor to orangutan and a hominoidancestor of gorilla, chimpanzee, andman; (ii) the emergence of the hominoidancestor; and (iii) the existence of aprogenitor of chimpanzee and man afterthe divergence of gorilla.
Cultured lymphocytes from two maleand three female orangutans (Pongo pyg-SCIENCE, VOL. 215, 19 MARCH 1982
Giesler, Jr., D. Menetrey, G. Guilbaud, J.-M.Besson, Brain Res. 118, 320 (1976).
12. J. C. Adams, Neuroscience 2, 141 (1977).13. L. A. Sternberger, Immunocytochemistry (Wi-
ley, New York, ed. 2, 1979).14. J. H. LaVail and M. M. LaVail, J. Comp.
Neurol. 157, 303 (1975).15. T. Hokfelt, L. Terenius, H. G. J. M. Kuypers,
0. Dann, Neurosci. Lett. 14, 55 (1979); R. M.BoWvker, H. W. M. Steinbusch, J. D. Coulter,Brain Res. 21l, 412 (1981).
16. J. H. Neale. J. L. Barker, G. R. Uhl, S. H.Snyder, Science 201, 467 (1978).
17. I thank J. Coffield and E. Humphrey for theirexpert technical assistance; L. Ostby for lightmicroscopic photography; M. Abdelmoumene,G. Bennett, P. J. P. Brickfield, R. Dubner, S.Gobel, and M. Hoffert for reading the manu-script; and E. Welty for typing it.
29 June 1981; revised 24 August 1981
maeus), one male and four female goril-las (Gorilla gorilla), four male and fivefemale chimpanzees (Pan troglodytes),and ten women and 21 men (Homo sapi-ens) were examined by use of a high-resolution chromosome-methotrexatecell synchronization technique (3). Totest for equivalence between chromo-somes and bands in the four species, wephotographed 20 relatively straight, G-bandedi late-prophase examples of eachchromosome from the four species(x 1600). The photographs were enlargedtwice and matched side by side- for adetailed analysis of reproducibility ofbanding patterns, band thickness, andstaining intensity (Fig. 1). Additionalchromosome preparations were stainedwith the C-banding technique (4) to de-terinine to what extent the banding pat-terns observed were related to hetero-chromatin. Descriptions were simplifiedby ascribing the new international hu-man high-resolution chromosome no-menclature (5) to the chromosomes ofthe great apes. Occasionally, when aquestion arose regarding chromosomalancestry of the four species, individualchromosomes from more primitive spe-cies were studied. These included twomale and two female rhesus monkeys(Macaca mulatta) and one male baboon(Papio papio). The results illustrate aremarkable similarity in the banding pat-terns of most chromosomes. Except fordifferences in nongenic constitutive het-
erochromatin (6), chromosomes 6, 13,19, 21, 22, and X appear to be identical inall four species; chromosomes 3, 11, 14,15, 18, 20, and Y look the same in threespecies; and chromosomes 1, 2p, 2q (7),5, 7 to 10, 12, and 16 are alike in twospecies (Figs. 1 and 2).Most chromosomal differences in the
four species consist of inversions ofchronmosomal segments and variations inconstitutive heterochromatin. The mostcommon inversions are of the pericentrictype, although a few are paracentric.Chromosomes 4, 5, 9, 12, 15, and 16 ofman and chimpanzee differ by a pericen-tric inversion, whereas chromosome 7 ofchimpanzee and gorilla differ by a para-centric inversion. Occasionally, bothperi- and paracentric inversions appearto be involved, as shown by comparisonsof chromosome 16 of man and gorilla andchromosomes 3 and 17 of man andorangutan.
Differences in constitutive heterochro-matin among the four species are causedby (i) variations in amount for the centro-meric and paracentromeric regions (par-ticularly in chromosomes 1, 9, 16, andthe short arm of acrocentric chromo-somes 13 to 15, 21, and 22) (6); (ii) thepresence of intercalary bands in chim-panzee (added to subbands 7q22.2 and13ql4.2) and orangutan (additional to thedistal end of band 4q12); and (iii) differ-ences in size of the Y chromosome,which tend to obscure the basic homolo-gy of the nonheterochromatic segment(pll.32q11.23) in the four species (Fig.2). In addition, telomeric or terminalbands are found in approximately half ofall the chromosome arms from chimpan-zee and in nearly all of those from goril-la, but they are conspicuously absent inchromosomes from man and orangutan(Fig. 2). Polymorphic variations of het-erochromatin were commonly found inthe four species and were particularlydramatic in the telomere of the short armof gorilla chromosomes 2q, 13 to 15, and18.
In addition to inversions and varia-tions in heterochromatin, a few chromo-somes of the four species showed recip-rocal translocation (5; 17 in gorilla), bandinsertion (terminal band 2OpI3 on centro-meric band 8q 1.2 in orangutan), differ-ences in the number and position ofnucleolar organizers (8), and telomericfusion (chromosomes 2p and 2q, withinactivation of the 2q centromere in man)(, 2). Humans have nucleolar organizerson chromosomes 13 to 15, 21, and 22; inchimpanzee, they are on chromosomes13, 14, 18, 21, and 22; in gorilla onchromosomes 13, 21, 22; and in orang-
Abstract. AMan, gorilla, and chimpanzee likely shared an ancestor in whom thefinegenetic organization of chromosomes was similar to that of present man. A com-parative analysis of high-resolution chromosomes from orangutan, gorilla, chim-panzee, and man suggests that 18 of23 pairs of chromosomes of modern man arevirtually identical to those ofour "common hominoid ancestor," with the remainingpairs slightly different. From this lineage, gorilla separated first, and three majorchromosomal rearrangements presumably occurred in a progenitor of chimpanzeeand man before the final divergence of these two species. A precursor of thehominoid ancestor and orangutan is also assumed.
20 21 22 Y XFig. 1. G-banded late-prophase chromosomes (1000-band stage) from man, chimpanzee, gorilla, and orangutan, arranged from left to right,respectively, to better visualize the extensive homology that exists among them. Heterochromatin is variable and particularly abundant in theparacentromeric region of human chromosomes 1, 9, and 16, the telomeres of chimpanzee and gorilla, the short arm of chromosomes 13 to 15 ofgorilla, and the Y chromosome.
Ak
SCIENCE, VOL. 2151526
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19 MARCH 1982 1527
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utan on 2p, 2q, 7, 9, 13 to 15, and 22 (Fig. species (6), we first reconstructed the2). The telomeric fusion of chromosomes most likely chromosome complement of2p and 2q accounts for the reduction of a presumed common ancestor of man,the 24 pairs of chromosomes of the great chimpanzee, and gorilla (Fig. 3). Thisapes to 23 in modem man. was arrived at by assigning to our "com-When heterochromatin is not consid- mon hominoid ancestor" a chromosomal
ered, man and chimpanzee have 13 pre- complement that most easily explainssumably identical chromosome pairs the ones found in the present species.(chromosomes 3, 6 to 8, 10, 11, 13, 14, 19 Representative examples of the compar-to 22, and XY); man and gorilla have ative analysis used in deriving such annine (chromosomes 3, 6, 11, 13, 19 to 22, ancestral chromosomal complement fol-and XY); and man and orangutan have low.eight (chromosomes 5, 6, 12 to 14, 19, 21, Chromosome 3 of man, chimpanzee,and 22). Furthermore, reversal of the and gorilla appear to be the same exceptsomewhat numerous inversions that for the presence of a very small amounthave taken place in the four species and of telomeric heterochromatin in gorilla.the few instances of translocation, inser- Thus, chromosome 3 of the hominoidtion, and fusion, would result in virtually ancestor resembled chromosome 3 of100 percent homology of all of the non- man, chimpanzee, and gorilla (Fig. 3)heterochromatic G-positive and G-nega- and more closely that of man and chim-tive bands. This homology includes a panzee (9). Assignment of other ances-correspondence in the thickness and col- tral chromosomes such as 1, 5, 6, 12 toor intensity of every band observed in 16, 18 to 22, and X was relatively simplethe four primates at the 1000-band stage since they could be deduced from a(Figs. 1 and 2). similar analysis of the chromosomes ofThe similarities of a large number of great apes and man (Figs. 2 and 3).
chromosomes and the relatively simple However, other chromosomes, such assteps needed to explain the chromo- 2, 4, 7, and 17, show differences thatsomal differences among the four species need explanation. Chromosome 7 ofmake it possible to speculate about the chimpanzee and man differ from that ofexistence of common ancestors of the gorilla mainly by a paracentric inversiongreat apes and man. Because human, of the long arm, which in turn differschimpanzee, and gorilla chromosomes from that of orangutan by a pericentricare very closely related to each other, inversion. Thus, chromosome 7 of theand the orangutan is a more primitive hominoid ancestor was like that of gorilla
Fig. 3. Presumed an- Humancestors of man, chim- la
panzee, gorilla, and Chimpanzee 17-..I.i1 2orangutan. In the 1295imiddle are illustrated
Gorilla 17. * H Human-chimpanzeethe chromosomes of a 16-14-12 1 H progenitorpresumed hominoid °s8'5__ 2p Cancestor whose chro-mosomes were large- HOGly similar to those of x HCGOhuman (H) and to a 22 HCGOgreat extent to chim- 20 HFCGpanzee (C), gorilla 19 HCGO(G), and orangutan 17 H(0). Before the diver- 16 HO
15 HGOgence of man and 14 HCchimpanzee took 13 HCG0place, their ancestor 12 FIOshared similar chro- 10 HCmosomes 2p, 7, and Orangutan 29 GO
S HO9 (human-chimpanzee 17 7 Gancestor). Man then 6 HCGO Hominoldancestor.
*11 5 HO ancestordiverged by fusion of 4 Hchromosomes 2p and \ 3 OCG2qintochromosome2 42 2p GOand by a small peri- 2q Cocentric inversion inchromosomes 1 and Hominoid-orang 3_7 -1 17 Y\18. By contrast, sev- precursor 0 0 0 RB 0en and nine major nonheterochromatic changes in individual chromosomes occurred inchimpanzee and gorilla, respectively. A presumed precursor of the hominoid ancestor andorangutan had the same chromosomes as the hominoid ancestor except for chromosomes 3, 7,10, and Y, which were similar to those of orangutan, and chromosome 17, which was like that ofrhesus (R) and baboon (B). From this precursor, orangutan diverged with changes inchromosomes 2q, 4, 8, 11, 17, and 20.
1528
since it represents an intermediate step.The ancestral chromosome 2p is be-lieved to have been similar to that oforangutan and gorilla, with a pericentricinversion accounting for chimpanzee 2p.The ancestral 2q, on the other hand,resembled that of gorilla and chimpan-zee, and human chromosome 2 can beexplained by fusion of a chimpanzee-like2p and the ancestral 2q (Fig. 2).Chromosome 17 of the hominoid an-
cestor was ascertained to be human-likebecause of the finding in gorilla of ahuman-like long arm of chromosome 17translocated to chromosome 5 and thefact that chromosome 17 in chimpanzeediffers by a pericentric inversion fromthat of man (Fig. 2). In chromosome 4,the bulk of the long arm (segmentq22qter) is identical in the four species,but the rest of the chromosome is not thesame in any two species. We have usedthe human chromosome 4 as the ances-tor because it is the only one from whichthe others can be derived by a simple,but differing, pericentric inversion or in-sertion (Fig. 2).The common ancestor of man, chim-
panzee, and gorilla had 24 pairs of chro-mosomes. Of these, 18 pairs were similarto those of present man, and 15 pairswere similar to those of chimpanzee,gorilla, and orangutan (Fig. 3). Gorillaemerged as a result of a pericentric in-version in chromosomes 4, 8, 10, 12, 14,and 16, a reciprocal translocation be-tween chromosomes 5 and 17, and aparacentric inversion involving the shortarm of chromosome 16 and a rearrangeddistal end of the long arm of chromo-some 1. Figure 3 suggests that gorillaseparated first and left behind a progeni-tor of chimpanzee and man whose chro-mosome 2p was similar to that of pres-ent-day chimpanzee and whose chromo-somes 7 and 9 were similar to those ofmodern man. This hypothesis is support-ed by chromosomal differences best ex-plained through "obligatory" intermedi-ary steps. For example, chromosome 9of orangutan and gorilla are acrocentrics,and the human chromosome 9 can beexplained more simply by pericentricinversion and acquisition of paracentro-meric heterochromatin from an originalorang-gorilla-like chromosome (Figs. 1and 2). In contrast, chimpanzee chromo-some 9 likely arose by a pericentricinversion of the chromosome 9 of thehuman-like ancestor and not directlyfrom the chromosome 9 of the orang-gorilla-like ancestor, since this wouldhave required an unusual double peri-centric inversion, with two offour break-points being the same as those giving riseto the human chromosome 9. Man and
SCIENCE, VOL. 215
chimpanzee have basically the same
chromosome 7, whereas the gorilla-likeancestral chromosome differs from themby a paracentric inversion. Evidence fora common ancestor ofman and chimpan-zee also comes from chromosome 2,since human chromosome 2 is most sim-ply explained by telomeric fusion of a
chimpanzee-like 2p chromosome and a
2q chromosome similar to that of chim-panzee and gorilla (Figs. 1 and 2). Thesefindings on chromosomes 2, 7, and 9together suggest a common ancestor ofhuman and chimpanzee. From this fore-father, man emerged after the formationof a small pericentric inversion in chro-mosomes 1 and 18 and the fusion ofchromosomes 2p and 2q to form thecharacteristic human chromosome 2.For present-day chimpanzee to appear,
however, seven major nonheterochro-matic changes had to occur (Fig. 3).Once chromosome complements had
been deduced for common ancestors ofman and chimpanzee, and of man, chim-panzee, and gorilla, it became possible todetermine the likely chromosome com-
plement of a presumed progenitor of ourhominoid ancestor and orangutan. Thiswas accomplished by comparison ofchromosomes of the hominoid ancestorwith those of orangutan and relatedprimitive apes (baboon and rhesus). Asdeduced from Figs. 2 and 3, such a
progenitor of orangutan and our homi-noid ancestor had 24 chromosome pairs,14'/2 of which were like those of our
hominoid ancestor and orangutan (chro-mosomes 1, 2p, 5, 6, 9, 12 to 16, 18, 19,21, 22, and X). Three and one-half of theremaining 91/2 were the same as those ofpresent-day orangutan (chromosomes 3,7, 10, and Y); five were like those of ourhominoid ancestor (chromosomes 2q, 4,8, 11, and 20); and one was like that ofrhesus and baboon (chromosome 17)(Figs. 2 and 3). Chromosomes 3, 7, 10,and Y were considered orangutan-likeand 2q and 8 as hominoid-like, since theywere the same in the more primitiverhesus and baboon, except that the cen-
tromere of orangutan chromosome 3 is at3p21.33 and those of rhesus and baboonare at 3q23. Chromosome 17 in theorangutan differs from the human-likehominoid chromosome 17 by a pericen-tric and a paracentric inversion. Sincebaboon and rhesus have a chromosome17 with the same long arm inversion as
that of orangutan, and the rest of thechromosome is like that of man, theprogenitor was assigned a rhesus-ba-boon-like chromosome 17. Chromosome11 of orangutan has no known counter-part in other primates. However, sincechromosome 11 of man, chimpanzee,
19 MARCH 1982
and gorilla are similar, and since theydiffer from those of rhesus and baboonby a simple pericentric inversion(p1 L.2q13.5), the orangutan chromosome11 is believed to have arisen after specia-tion through a large pericentric inver-sion, followed by translocation of thecentromeric region. Finally, chromo-some 20 of orangutan probably arose
after speciation by a paracentric inver-sion in the long arm and insertion of theterminal band p13 into qI 1.21 of chromo-some 8.
Earlier work with metaphase chromo-somes had shown a basic similarityamong New and Old World monkeys,apes, and man, making it possible toconstruct a chromosomal phylogenyfrom prosimians to man (10). Althoughthe karyotype of the ancestor to man,
chimpanzee, and gorilla had beendeduced (2), several uncertainties re-
mained. The hominoid ancestral chro-mosome 1 had been classified as beingthe same in the four species instead ofbeing like that of chimpanzee and orang-
utan because with contracted chromo-somes it had not been possible to deter-mine a rearrangement of part of band q42at the telomeric end of the long arm
of the gorilla chromosome or to notea small pericentric inversion in that ofman. Chromosome 4 had been classifiedas being like that of gorilla and orang-
utan, when indeed it is different in allfour species. Chromosome 8 had beenclassified as being like that of man, chim-panzee, and orangutan rather than likethat of man and chimpanzee, since an
extra large G-negative band near thecentromeric region of the long arm oforangutan chromosome 8 was not detect-ed. Finally, the Y chromosome was be-lieved to differ in each species, andno homology could be found. Yet whenfinely banded chromosomes are usedand only the noncentromeric and nontel-omeric heterochromatic regions of the Ychromosome are considered (segmentpll.32qII.23), a basic homology is ob-served in man, chimpanzee, and gorilla(Figs. 1 and 2). The orangutan chromo-some Y can then be explained as possi-bly differing by a pericentric inversion(pl 1.2ql 1.23).
It was not possible earlier to derive a
complete ancestral karyotype of the pro-
genitor of the hominoid ancestor andorangutan because of technical difficul-ties in tracing evolutionary changes thatoccurred in chromosomes 3, 4, 8, 11, 17,20, and Y (2, 9). Also, because of appar-ent similarities in chromosomes 12 and16 and in telomeric heterochromatin ofchimpanzee and gorilla, no ancestor ofman and chimpanzee was considered (2,
9), and a hybrid zone was believed tohave existed for gorilla and chimpan-zee (2). Our refined banding techniqueshows that chromosomes 12 and 16of chimpanzee and gorilla were formedby different inversions (breakpoints inbands p11.23 and q14.2 in gorilla andp12.2 and ql5 in chimpanzee), and thereare differences in amount, color intensi-ty, and location of telomeric heterochro-matin in the two species (Figs. 1 and 2).With the use of high-resolution chro-
mosome technology, we can account forevery nonheterochromatic band of thethree great apes and man and have amore precise delineation of the structuralchromosomal rearrangements found inthe four species. The 100 percent homol-ogy of nonheterochromatic bands ofmanand the great apes is not surprising, sincemore than 50 genes have been located onhomologous chromosomes and chromo-some bands of the four species (11).
Studies of DNA reassociation kinetics(12), protein structure and antigenicity(13), and histocompatibility antigens andblood groups (14) all indicate that chim-panzee, gorilla, and man share a substan-tial common ancestry and that orangutandiverged earlier from this lineage. Fur-thermore, almost total homology of sin-gle-copy DNA (12) and amino acid se-quence of proteins (15) has been found inman and chimpanzee, suggesting a veryclose evolutionary relationship betweenthe two. These findings conflict with theview, based primarily on anatomic andbehavioral data, that man (placed in Ho-minidae family) diverged separately fromthe evolutionary line leading to the greatapes (placed in Pongidae family) (16).Our detailed comparative analysis ofhigh-resolution chromosomes supportsmolecular evidence that the great apesand man belong to the Hominidae fam-ily, which separates into the Ponginae(orangutan) and Homininae (gorilla,chimpanzee, and man) subfamilies (16).It also provides evidence in favor of theexistence of three ancestors to the greatapes and man from which first orang-utan, then gorilla, and finally chimpan-zee and man diverged.
JORGE J. YUNISOM PRAKASH
Department of Laboratory Medicineand Pathology, University ofMinnesota Medical School,Minneapolis 55455
References and Notes
1. J. de Grouchy et al., Ann. Genet. 15, 79 (1972);B. Dutrillaux, M. 0. Rethor6, J. Lejeune, ibid.18, 153 (1975); Paris Conference (1971), supple-ment (1975); Cytogenet. Cell Genet. 15, 201(1975).
2. B. Dutrillaux, "Sur la nature et l'origine deschromosomes humains," Monogr. Ann. Genet.Expansion Sci. Fr. (1975), pp. 41-71.
1529
3. J. J. Yunis, Science 191, 1268 (1976).4. , L. Roldan, W. G. Yasmineh, J. C. Lee,
Nature (London) 231, 532 (1971).5. J. J. Yunis, J. Hum. Pathol. 12, 494 (1981).6. __ and W. G. Yasmineh, Science 174, 1200
(1971); J. R. Gosden, A. R. Mitchell, N.Seuanez II, C. M. Gosden, Chromosoma 63, 253(1977).
7. The short arm (p) and long arm (q) of humanchromosome 2 are represented by two acrocen-tric chromosomes (2p and 2q) in the great apes.
8. R. Tantravahi, D. A. Miller, V. G. Dev, 0. J.Miller, Chromosoma 56, 273 (1978).
9. Since constitutive heterochromatin representsnongenic material that can be acquired afterspeciation and often varies from species to spe-cies (6), it was not included in the analysisillustrated in Fig. 3.
10. B. Dutrillaux, Humangenetik 48, 251 (1979).11. Human Gene Mapping 5, Cytogenet. Cell Ge-
net. 25, 82 (1979).
Exposure to a variety of stressors pro-duces a subsequent decrease in pain re-sponsiveness (1). This stress-induced an-algesia has received considerable recentattention, largely because of its potentialfor providing insight into a possible func-tional role for endogenous opioids inbehavioral and adaptive phenomena.The brain has pain-inhibiting systems inwhich endogenous opioids may play arole (2). The phenomenon of stress-in-duced analgesia suggested that endoge-nous opioids might be released by stress,thereby inhibiting pain and perhaps pro-tecting the organism in some way (3).More recent work, however, has sug-gested that some forms of stress-inducedanalgesia are mediated by opioid sys-tems [animals develop a cross-tolerancebetween the analgesic effects of mor-phine and the stress, and the analgesiacan be reversed by opiate antagonists(4)], whereas others are mediated bynonopioid mechanisms (5).
Since multiple opioid systems exist inboth the brain and the pituitary (6), re-cent research has been directed at deter-mining which system mediates the opioidform of stress-induced analgesia. Thediscovery that hypophysectomy (remov-al of the pituitary) reduces an opiate-mediated stress-induced analgesia hassupported the proposal that pituitary ,B-endorphin may mediate this type of anal-gesia (1). In further support of this view,the synthetic glucocorticoid dexametha-
12. P. L. Deininger and C. W. Schmid, Science 194,846 (1976); R. E. Benveniste and G. T. Todaro,Nature (London) 261, 101 (1976).
13. M. Goodman and R. E. Tashian, Eds, Molecu-lar Anthropology (Plenum, New York, 1976).
14. W. W. Socha and J. Moor-Jankowski, J. Hum.Evol. 8,453 (1979); J. J. Garver, A. M. Estop, P.Meera Kahn, H. Balner, P. 0. Pearson, Cyto-genet. Cell Genet. 27, 238 (1980).
15. M.-C. King and A. C. Wilson, Science 188, 107(1975).
16. A. L. Zihiman, J. E. Cronin, D. L. Cramer, V.M. Zarich, Nature (London) 275, 744 (1978); M.Goodman, Prog. Biophys. Mol. Biol., in press.
17. Supported in part by NIH grant 26800. We thankthe Yerkes Primate Center of Georgia for gorillaand orangutan blood samples, K. Dunham ofAmes, Iowa, for chimpanzee blood samples,and A. Beacom for reviewing the manuscript.
23 July 1981; revised 10 December 1981
sone has been shown to block both thestress-induced rise in plasma ,3-endor-phin (7) and opioid stress-induced anal-gesia (5).These and other findings led Baizman
et al. (8) and Lewis et al. (5) to proposethat pituitary ,-endorphin might be mo-bilized by stress transported to the brainby retrograde flow through the portalsystem, and thus decrease responsive-ness to painful stimuli by interacting withcentral structures. Such an action bypituitary P-endorphin is possible, but dif-ficult to reconcile with reports that thehypothalamic content of f-endorphin de-creases rather than increases after 30minutes offootshock, and that even largedoses of intravenous ,B-endorphin havelittle effect on pain responsiveness (9).Both hypophysectomy and dexametha-sone treatment either eliminate or reducethe stress-induced release of pituitaryadrenocorticotropic hormone (ACTH) aswell as ,-endorphin (7). This fact isnoteworthy because corticosterone isunder anterior lobe ACTH regulation,and corticosterone affects central pro-cesses associated with pain inhibition(10, 11). This line of reasoning suggeststhat the manipulations which seem toimplicate pituitary P-endorphin may ac-tually produce their effects by alteringpituitary-adrenocortical interaction.The purpose of our studies was to
examine the role of the pituitary-adrenalaxis in the production of the opioid form
0036-8075/82/0319-1530$01.00/0 Copyright K 1982 AAAS
of stress-induced analgesia. A number ofdifferent procedures result in an opioidstress-induced analgesia. We used a pro-cedure in which subjects are not testeduntil 24 hours after the stress session,but the pain responsivity test is precededby a brief reinstatement procedure inwhich the subject is again exposed to thestressor (4). This allows for the dissipa-tion of nonspecific factors such as fa-tigue and local anesthetic effects andmay result in a "purer" opioid form ofanalgesia. The long-term effect is com-pletely reversed by opiate antagonistsand completely cross-tolerant with mor-phine, whereas these outcomes are notalways complete under short-term test-ing soon after the stress session, even ifthe stress session is prolonged (5).We first examined the effects of hy-
pophysectomy on long-term analgesia.Eight hypophysectomized rats and eightthat had been subjected to sham surgery(12) were restrained and treated with 805-second 1-mA shocks delivered throughfixed tail electrodes on the average ofone per minute. Eight more rats of eachtype were restrained for an equivalentperiod, but not shocked. Twenty-fourhours later all rats received a shockreexposure procedure which consistedof five single-crossing shuttlebox escapetrials. Immediately afterward, subjectswere given three analgesia test trials (at4-minute intervals) in a tail-flick appara-tus in which latency to flick the tail fromradiant heat served as the measure ofpain sensitivity. These procedures aredescribed elsewhere (4). Hypophysecto-my completely blocked the long-termanalgesia effect (Fig. lA). A 2 by 2analysis of variance confirmed this con-clusion by showing significant shock[F(1, 28) = 13.24, P < .01] and hypoph-ysectomy [F(1, 28) = 12.13, P < .01]main effects, and, most important, a sig-nificant interaction of hypophysectomywith shock [F(1, 28) = 10.66, P < .01].Newman-Keuls individual group com-parisons (a = .05) indicated that the in-escapably shocked sham rats were moreanalgesic than the other three groups.
In a second experiment, a 2 by 4factorial design was used with rats re-strained or inescapably shocked 2 hoursafter receiving an intraperitoneal injec-tion of dexamethasone [0 mg per kilo-gram ofbody weight (saline), 0.25 mg/kg,0.5 mg/kg, or 1.0 mg/kg]. As in the firstexperiment, 24 hours later all rats weregiven five shuttlebox trials and threeanalgesia test trials.Dexamethasone prevented the ines-
capable shock-induced analgesia (Fig.IB), with blockage complete at 02i and1.0 mg/kg. Newman-Keuls individual
SCIENCE, VOL. 215, 19 MARCH 1982
Corticosterone: A Critical Factor in anOpioid Form of Stress-Induced Analgesia
Abstract. The finding that some opioid-mediated forms of stress-induced analge-sia are antagonized by hypophysectomy and dexamethasone has led to the sugges-tion that f-endorphin, released from the pituitary, may mediate these analgesicreactions. "Long-term analgesia" (an opioid-mediated form of stress-inducedanalgesia), which is blocked by dexamethasone and hypophysectomy, was alsoblocked by adrenalectomy and reinstated with corticosterone therapy. Corticoster-one is proposed to play a permissive role in long-term analgesia and to be a criticalhormone mediating this phenomenon.
