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β-endorphin: Dose-dependent suppression of fixed-ratio operant behavior
Pergamon Preae
Life Sciences, Vol. 21, pp . g27-g32 Printed in the U .SA.
ß-BNDORPHIMs
DOSS-
BûPPR88SI0ti OF FI7~D-RATIO OPSR7IN'P HBBAVIOR
Leonard Lichtblau, Linda H. Foasam and Shsldon H . Sparbar Department of Pha~acology, University of Minnewta Minneapolis, Minnesota 55455 (Received in final fora August 23, 1977) Centrally administered ß-endorphin or morphine suppressed fixed-ratio 15, food-reinforced responding by rate is a dosedspsndaat meaner . ß-Endorphin was 21 times more potent than morphine on a molar basis. Scratching and wet-dog shakes were observed within 30 minutes of ß-endorphin administration but were not aeon after morphine and did not appear to be reapoasibla for the supprassioa of the ooaditioaed behavior . Recent reports have demonstrated that ß-endorphin, a pituitary peptide (1), possesses a number of properties ao®on to opiates . It has base shown to prodace analgesia (2-7), physical dependence and tolerance (2,5,7,8), as wall as cross physical dependence on the cross tolerance to sorphiae (2,9) . 1ldditionally, the action of ß-endorphin can be reversed or blocked by the spacilic opiate aatagoaist, nalozoas (3,5,9,10) . On a molar basis, ß-endorphin is reported to ba 10-50 times more potent than morphine, using unconditioned behavioral msaauraa of analgesia (3-5,9) . This peptide has been reported to produce a profound cataleptic state, loafing up to several hours, is opiate-naive animals (4,6,10) . In addition, wet-dog shakes, typical of opiate withdrawal, have base observed following rel atively large doses of ß-endorphin administration (10) . To date, the effect of thin peptide on conditioned behavior has not been reported . Operant wnditioaed behavior is ssasitivs to disryption by a number of drugs (11,12) including the opiates (13,14) . lie have a:amined and compared the affects of centrally administered B-~adorphin sad morphine on fixed-ratio re sponding for food reinforcement. Our findings indicate that as little as 0.7 nmole (2 .4 Lq) of ß-endorphin will disrupt fixed-ratio responding 50 " during 30 minute behavioral sessions . lie also report that ß-endorphin is 21 tines more potent than morphine, on a molar basis, in altering this conditioned behavior . Materials and Methods Tea, eight-month old, Long-8vans male hooded rata (Simonsen Laboratorieai Gilroy, Calif.) weighing approximately 475-600 gm served as ezperimental subjects . They were housed individually is fe~perature (25oC) sad humidity (501) controlled quarters with a 12 hour light-dark cycle. After being food-deprived to 70-801 of their free-fe~diag weights the rate were shaped to lever-press oa a fixed-ratio 15 (FR 15) schedule of food rsinforoestsat . FR 15 schedules of lever-pressing require the experimental subject to depress a lever 15 tines for the delivery of one rainforcar (45 mg food psllett P.J . Noyes Co ., Lancaster, M.H .) . Hater was available ad libitwa in their home cages . These rats had 927
928
ß-Endorphin and Conditioned Behavior
Vol . 21, No . 7, 1977
received one or two injections of morphine (3 .75-7 .5 mg/kg, i .p .) in a previous study, but were drug-free for five months prior to the start of the present experiment . Rats were anesthetized with sodium pentobarbital (45 mq/kq) and mounted on a atereotaxic apparatus . Chronic indwelling, 23 gauge stainless steal infusion cannula guides were implanted into the left or right lateral ventricles at coor dinates determined from a standard stereotaxic atlas of the rat brain (15) . A detailed description of this methodology is found elsewhere (16,17) . Behavioral sessions were conducted in standard operant chambers contained within light and sound attenuating environments . Each environment was equipped with a closed-circuit video system, allowing continuous observation of the rata' gross behavior during the sessions, without disturbing them . The experimental sessions were controlled by and data recorded in a minicomputer (NOVA 2/10, Data General Corp ., Southboro, Mass .) interfaced with the operant chambers by Interact hardware and software (BRS/LVE, Beltsville, Md .) . In addition, a cumulative record of each behavioral session was made . ß-Endorphin (M .W . 3438) was synthesized as previously described (18) and was a generous gift of Dr . C . H . Li . Morphine sulfate (M .W . 758) was purchased commercially . Doses of morphine are given in terms of the free base (M .W . 285) . Drug solutions were prepared fresh daily (ß-endorphin from lyophilized aliquots) so that 10 ul of sterile isotonic saline would contain the desired dose . Two to three weeks prior to the start of the experiment, infusion cannulas were implanted into the lateral ventricles of rats previously shaped to leverpreas on a FR 15 schedule of food reinforcement . Experimental sessions lasted thirty minutes during which the number of reinforcers received by the rats was recorded . After baseline responding had stabilized, the subjects were distributed into groups, to be given the various doses of drugs, using a randomized block design for response rates . In addition, each group had subjects implanted in either right or left lateral ventricles . Prior to drug sessions rats were habituated to intracerebroventricular (i .c .v .) amine infusions, the last of which served as control for the drug data . Rata were infused, i .c .v ., with 0 .3, 1 .5 or 2 .9 nmoles (1, 5 or 10 ug, respectively) of ß-endorphin in a 10 ul volume, over a sixty aeconfl interval, and placed in the behavioral chamber . In addition to recording the number of reinforcers earned by the rat for the thirty minute session, the behavioral chamber remained operative until a total of 50 reinforcers had been earned, so that an estimate of the duration of ß-endorphin's action might be made . A least squares linear regression analysis was performed on the doaereaponee data and a dose of ß-endorphin which suppressed responding during the thirty minute drug session by 50i (ED50) was estimated . This dose of ß-endor phin was later (2-3 weeks) administered to the rats . To determine the relative potency of ß-endorphin to morphine, rats were infusefl, i .c .v ., (3-4 weeks after the last dose of ß-endorphin) with 5 .3, 14 or 29 nmolea (1 .5, 4 .1 or 8 .3 uq, respectively) of morphine . Additionally, based on analysis of the beat fitting straight line, determined by a least squares linear regression, a dose of morphine which suppressed responding in the thirty minute drug session by 33i (ED33) was estimated and administered to the rata 2-3 weeks later . The ED33 was chosen, rather than the ED50, as had been done for ß-endorphin, because a doss of morphine which suppressed responding by 50! had already been administered as part of the dose-response experiment . Between the time of the initial estimation of the dose-response curve for ß-endorphin and the terminal verification of the estimated ED33 of morphine, 3 rate
92 9
ß-Endorphin sad Conditioned Behavior
Vol . 21, No . 7, 1977
ware deleted from the experiment . Two rats developed severe respiratory infections and one rat became too agqresaive and difficult to handle to inject easily . Results and Discussion During the thirty minute drug session, ß-endorphin's suppression of fixedratio responding was dose-dependent (Fig . 1) . Administration of the three doses of the peptide also produced an increase in the time required to earn 50 rein forcers . Under control condition, the rata took 0 .11 t 0 .01 hr to reach this criterion . when 0 .3, 1 .5 and 2 .9 nmoles of the peptide were administered, the criterion was reached after 0 .19 t 0 .02, 0'.58 t 0 .22 and 3 .71 t 1 .92 hr (M . t S .E .M.) respectively . Analysis of the log-transformed oalues of the duration of action data revealed a statistically significant effsat (F ~ 17 .97 for 1 and 8 df, P < .005) .
100
ENDORPHIN
MORPHINE
S é $ 80 w 0
(2)
(7)
g 60 c E ~ 40 _c a ~ 20 0a a 0 o:
QL Q,I
ED50=14.4 nmole~ (8.3-24.8)
Y=-28.9 (In X)+37.5 ~
i
v
v wii L0
v
(d i i
~
DOSE (nmoles)
(3)
Y=-29.9(In X)+l29.8
wii 10 .0
v
v
v
i LiLu
100 .0
FIG . 1 The effect of intracerebroventricularly administered ß-endorphin or morphine on FR 15 lever-pressing in rats . Data are presented ae Mean t 3 .E .M . percent of control responding for each done of drug administered . S .E .M . e are not presented if less than the radius of the deta point . The number of animals in each q;oup is noted in parentheses . Under control conditions (saline i .c .v .) animals averaged 202 t 12 (Mesa t S .E .M ., n ~ 10 animals) reinforcers per 30 min session . Open figures (A-ß-endorphins o-morphine) shoo data from the initial dose-response experiments . Closed fiqgures show the data from subsequent infuaiona (see text) . Equations are given for the regression line for each drug determined by parallel line bioassay (20) . EDSOs (and 95ti confidence intervals) are given in the figure . Least squares linear regraasion analyses of log-transformed data revealed significant negative regressions between the dose of either ß-endorphin (F ~ 23 .36 for 1 and 8 df, P < .005) or morphine (F ~ 8 .38 for 1 and 6 df, P < .OS) and the rate of responding .
430
ß-Endorphin and Conditioned Behavior
Vol . 21, No . 7, 1977
11s can be seen in Figure 1, 5 .3, 14 and 29 nmoles of morphine produoed a dose-related suppression in fixed-ratio responding over the thirty minute ezperimental session . The ED50 of morphine, under these conditions is 14 .4 nmole . On a molar basis, ß-endorphin is 21 times more potent than morphine in auppreseinq fixed-ratio responding . Cochin and ICOrneteky (19) have reported that prior treatment pith morphine may inducé long lasting changes (tolerance) in the analgesic response of rata If such changes also occur following ß on subsequent exposure to morphine . endorphin, the dose-response relationships and hence the relative potencies of ß-endorphin and morphine may have been altered by repeated drug administration . However, this does not appear to be the case . The disruptive effect, on fixedratio responding, of the estimated ED50 of ß-endorphin (0 .7 nmole) and SD33 of morphine (8 .4 nmole) were not different than predicted, i .e . 47 t 14t and 33 t 13~ respectively (Fig . 1) . Since, under these experimental conditions, prior exposure to ß-endorphin or morphine did not alter the response of subjects to a second exposure to the same compound, within 2-3 weeks, it ae~e unlikely that ß-endorphin altered the response of rata to subsequent exposure to morphine and even more unlikely that a single exposure to morphine 5 months previously would have altered the response of the rats to ß-endorphin .
Rat L~7
Saline
N
m
Q{
O O v
ß-Endorphin (0 .7 nmoles)
Morphine (8.4 nmoles)
30 Minutes FIG . 2 Sample cumulative records for rat L-7 showing FR 15 responding immediately after i .c .v . infusion of 10 ul saline, 0 .7 nmole ß-endorphin or 8 .4 nmole morphine . Responding rate is reflected by the elope of the recording . Delivery of a reinforcer is indicated as a pip on the ascending record .
Vol . 21, No . 7, 1977
ß-Hndorphia sad Conditioned Behavior
931
Visual analysis of the behavioral sessions revealed that during the period in which the rats wars not lever-pressing thsy oitea engaged in behavior which varied with the dop of ß-enWorphin administered. Following the lowest dose (0 .3 nmoie) of ß-endorphin, the rats ware observed to engage is multiple boute of head and ear scratching . Scratching occurred bilatarally and was not related to the particular ventricle infused with peptide. At 1 .5 nmols, occasional wet-dog shakes appeared, in addition to the scratching . At 2 .9 nmola, acratchinq and considerably more wet-dog shaken (over 125 in one rat) occurred throughout the thirty ainute session. Scratching and wet-dog shakes were not seen lollowiag saline or morphine infusions . It should be pointed out that the amount of time spent scratching and/or shaking was only a fraction of the total time the rats ware not lever-pressing and could not account for the disruption in this behavior (except perhaps at the lowest dose) . Figure 2 shows examples of cumulative records of lever-pressing behavior alter injection of saline, ß-endorphin or morphine i .c .v . Although short bouts of scratching occurred after ß-endorphin but not alter morphine, the pattern of responding after both drugs looks quite similar. Pausing after delivery of the reinforcvrs ie increased, while running rates (respondinq during bursts or rune after postreinforcemant pauses) remain relatively constant and atippear identical to running rates during the saline session. . The highest dosaa of ß-endorphin and morphine produced an abrupt cessation of responding, typically immediately after delivery of a reinforcer . From these experiments it is seen that= (1) Centrally administered ß-endorphin and morphine suppress fixed-ratio operant behavior in a dose-dependant manner . (2) On a molar basis, ß-endorphin is 21 times more potent than morphine in its capacity to suppress this behavior . (3) Tolaraaoe to the behavioral suppressant action of either drug is not seen 2-3 weeks following an initial exposure . (4) Scratching and wet-dog shakes are seen only after ß-endorphin but do not entirely acwunt for the disruption in the behavior . To date, most investigations haw dealt with the similarities of ß-endorphin and morphine . However, in light of the differences between the two (catatonia (4,10), scratchin~ and wet-dog shakes seen only after ß-endorphin) future iavestigation to nocount for these differences is warranted. AçknoaledQ~nt This research was supported in part by USP~B Grant DA-00532 . References C. H. Li and D. Chunq, Froc . Natl . Aced . Sci. 73 1145-1148 (1976) . J. eläsiq and A. Hors, Nauayn-Schmiedeberq's Arch . Phaxmacol. 294 297-300 (1976) . 3. L. Graf, J. I. Szekely, A. Z . Roam, Z. Durai-1COVacs and S . Bajusz, Nature 263 240-241 (1976) . 4. Y .F. Sacquat and N . Marks, Science 194 632-635 (1976) . 5 .~. H. Loh, L. F . Tsenq, E . 9iei and C . H. Li, Pzoc . Natl . Aced . Sci . _73 2895-2898 (1976) . 6. H. Teschamachsr, J. Bläsiq and Vi . Bramer, Naunyn-Schmiedaberq's Aroh . Pharmacol . _294 293-295 (1976) . 7. J. M. van Ree, D. Deified, A. F . Bradbury, E. C . Hulme, D. C . Smyth and C. R. Snell, Nature 264 792-794 (1976) . 8. E. wei and H. Loh, Scieras _193 1262-1263 (1976) . 9 . L. F. Tsenq, H. H. Loh sad C. H. Li, Proc . Natl . Aced . Sci. _73 4187-4189 (1976) . F. E. Bloom, D. 8ega1, N. Longard sad R. Guilleman, Science _194 630-632 (1976) . 1. 2.
932 11 . 12 . 13 . 14 . 15 . 16 . 17 . 18 . 19 . 20 .
ß-Endorphin aad Conditioned Behavior
Vol . 21, No . 7, 1977
R . T . Reueher and W . H . Morse, Ergeb . Physiol . _60 1-56 (1968) . T . Thampaon and J . J . Horen, Handbook of Operant Behavior , W . R . Honig and J . E . R. Staddon, Eds ., pp . 540-569, Prentice-Hall, Englewood Cliffs, N .J . (1977) . D . E . McMillan, Adv . Hiochem . Psychopharmacol . 8 345-359 (1973) . S . B . Sparbar, V . F . Gallert, L . Lichtblau and R. Eisenberg, Sympoai~an on Factors Affecting the Action of Narcotics , Milan, Italy, July 1976, M . W . Adler, L . Marsara and R . Samanin, Eds ., in press . J . E . Skinner, Neuroscience : A Laboratory Manual , pp . 195-237, W . H . Saun dera, Philadelphia, Pa . (1971) . S . B . Sparher, DaCI Carrier _2 1-5 (David Kopf Instruments, Tujunga, Calif ., 1975) . H . A . Tilson, R . H . Rech and S . B . Sparher, Pharmacol . Biochem . Behau . _3 385-392 (1975) . C . H . Li, 3 . Lemaire, D . Yamashiro and B . A . Doneen, Biochem . Biophys . Rea . C~un . _71 19-25 (1976) . J . Cochin and C . Rornetsky, J . Phas~acol . Exp . Ther . 145 1-10 (1964) . D . J . Finney, Statistical Methods in Hioloqical Assay, 2nd ed ., Hafner Publishing, New York, N .Y . (1964) .
Life Sciences, Vol. 21, pp . g27-g32 Printed in the U .SA.
ß-BNDORPHIMs
DOSS-
BûPPR88SI0ti OF FI7~D-RATIO OPSR7IN'P HBBAVIOR
Leonard Lichtblau, Linda H. Foasam and Shsldon H . Sparbar Department of Pha~acology, University of Minnewta Minneapolis, Minnesota 55455 (Received in final fora August 23, 1977) Centrally administered ß-endorphin or morphine suppressed fixed-ratio 15, food-reinforced responding by rate is a dosedspsndaat meaner . ß-Endorphin was 21 times more potent than morphine on a molar basis. Scratching and wet-dog shakes were observed within 30 minutes of ß-endorphin administration but were not aeon after morphine and did not appear to be reapoasibla for the supprassioa of the ooaditioaed behavior . Recent reports have demonstrated that ß-endorphin, a pituitary peptide (1), possesses a number of properties ao®on to opiates . It has base shown to prodace analgesia (2-7), physical dependence and tolerance (2,5,7,8), as wall as cross physical dependence on the cross tolerance to sorphiae (2,9) . 1ldditionally, the action of ß-endorphin can be reversed or blocked by the spacilic opiate aatagoaist, nalozoas (3,5,9,10) . On a molar basis, ß-endorphin is reported to ba 10-50 times more potent than morphine, using unconditioned behavioral msaauraa of analgesia (3-5,9) . This peptide has been reported to produce a profound cataleptic state, loafing up to several hours, is opiate-naive animals (4,6,10) . In addition, wet-dog shakes, typical of opiate withdrawal, have base observed following rel atively large doses of ß-endorphin administration (10) . To date, the effect of thin peptide on conditioned behavior has not been reported . Operant wnditioaed behavior is ssasitivs to disryption by a number of drugs (11,12) including the opiates (13,14) . lie have a:amined and compared the affects of centrally administered B-~adorphin sad morphine on fixed-ratio re sponding for food reinforcement. Our findings indicate that as little as 0.7 nmole (2 .4 Lq) of ß-endorphin will disrupt fixed-ratio responding 50 " during 30 minute behavioral sessions . lie also report that ß-endorphin is 21 tines more potent than morphine, on a molar basis, in altering this conditioned behavior . Materials and Methods Tea, eight-month old, Long-8vans male hooded rata (Simonsen Laboratorieai Gilroy, Calif.) weighing approximately 475-600 gm served as ezperimental subjects . They were housed individually is fe~perature (25oC) sad humidity (501) controlled quarters with a 12 hour light-dark cycle. After being food-deprived to 70-801 of their free-fe~diag weights the rate were shaped to lever-press oa a fixed-ratio 15 (FR 15) schedule of food rsinforoestsat . FR 15 schedules of lever-pressing require the experimental subject to depress a lever 15 tines for the delivery of one rainforcar (45 mg food psllett P.J . Noyes Co ., Lancaster, M.H .) . Hater was available ad libitwa in their home cages . These rats had 927
928
ß-Endorphin and Conditioned Behavior
Vol . 21, No . 7, 1977
received one or two injections of morphine (3 .75-7 .5 mg/kg, i .p .) in a previous study, but were drug-free for five months prior to the start of the present experiment . Rats were anesthetized with sodium pentobarbital (45 mq/kq) and mounted on a atereotaxic apparatus . Chronic indwelling, 23 gauge stainless steal infusion cannula guides were implanted into the left or right lateral ventricles at coor dinates determined from a standard stereotaxic atlas of the rat brain (15) . A detailed description of this methodology is found elsewhere (16,17) . Behavioral sessions were conducted in standard operant chambers contained within light and sound attenuating environments . Each environment was equipped with a closed-circuit video system, allowing continuous observation of the rata' gross behavior during the sessions, without disturbing them . The experimental sessions were controlled by and data recorded in a minicomputer (NOVA 2/10, Data General Corp ., Southboro, Mass .) interfaced with the operant chambers by Interact hardware and software (BRS/LVE, Beltsville, Md .) . In addition, a cumulative record of each behavioral session was made . ß-Endorphin (M .W . 3438) was synthesized as previously described (18) and was a generous gift of Dr . C . H . Li . Morphine sulfate (M .W . 758) was purchased commercially . Doses of morphine are given in terms of the free base (M .W . 285) . Drug solutions were prepared fresh daily (ß-endorphin from lyophilized aliquots) so that 10 ul of sterile isotonic saline would contain the desired dose . Two to three weeks prior to the start of the experiment, infusion cannulas were implanted into the lateral ventricles of rats previously shaped to leverpreas on a FR 15 schedule of food reinforcement . Experimental sessions lasted thirty minutes during which the number of reinforcers received by the rats was recorded . After baseline responding had stabilized, the subjects were distributed into groups, to be given the various doses of drugs, using a randomized block design for response rates . In addition, each group had subjects implanted in either right or left lateral ventricles . Prior to drug sessions rats were habituated to intracerebroventricular (i .c .v .) amine infusions, the last of which served as control for the drug data . Rata were infused, i .c .v ., with 0 .3, 1 .5 or 2 .9 nmoles (1, 5 or 10 ug, respectively) of ß-endorphin in a 10 ul volume, over a sixty aeconfl interval, and placed in the behavioral chamber . In addition to recording the number of reinforcers earned by the rat for the thirty minute session, the behavioral chamber remained operative until a total of 50 reinforcers had been earned, so that an estimate of the duration of ß-endorphin's action might be made . A least squares linear regression analysis was performed on the doaereaponee data and a dose of ß-endorphin which suppressed responding during the thirty minute drug session by 50i (ED50) was estimated . This dose of ß-endor phin was later (2-3 weeks) administered to the rats . To determine the relative potency of ß-endorphin to morphine, rats were infusefl, i .c .v ., (3-4 weeks after the last dose of ß-endorphin) with 5 .3, 14 or 29 nmolea (1 .5, 4 .1 or 8 .3 uq, respectively) of morphine . Additionally, based on analysis of the beat fitting straight line, determined by a least squares linear regression, a dose of morphine which suppressed responding in the thirty minute drug session by 33i (ED33) was estimated and administered to the rata 2-3 weeks later . The ED33 was chosen, rather than the ED50, as had been done for ß-endorphin, because a doss of morphine which suppressed responding by 50! had already been administered as part of the dose-response experiment . Between the time of the initial estimation of the dose-response curve for ß-endorphin and the terminal verification of the estimated ED33 of morphine, 3 rate
92 9
ß-Endorphin sad Conditioned Behavior
Vol . 21, No . 7, 1977
ware deleted from the experiment . Two rats developed severe respiratory infections and one rat became too agqresaive and difficult to handle to inject easily . Results and Discussion During the thirty minute drug session, ß-endorphin's suppression of fixedratio responding was dose-dependent (Fig . 1) . Administration of the three doses of the peptide also produced an increase in the time required to earn 50 rein forcers . Under control condition, the rata took 0 .11 t 0 .01 hr to reach this criterion . when 0 .3, 1 .5 and 2 .9 nmoles of the peptide were administered, the criterion was reached after 0 .19 t 0 .02, 0'.58 t 0 .22 and 3 .71 t 1 .92 hr (M . t S .E .M.) respectively . Analysis of the log-transformed oalues of the duration of action data revealed a statistically significant effsat (F ~ 17 .97 for 1 and 8 df, P < .005) .
100
ENDORPHIN
MORPHINE
S é $ 80 w 0
(2)
(7)
g 60 c E ~ 40 _c a ~ 20 0a a 0 o:
QL Q,I
ED50=14.4 nmole~ (8.3-24.8)
Y=-28.9 (In X)+37.5 ~
i
v
v wii L0
v
(d i i
~
DOSE (nmoles)
(3)
Y=-29.9(In X)+l29.8
wii 10 .0
v
v
v
i LiLu
100 .0
FIG . 1 The effect of intracerebroventricularly administered ß-endorphin or morphine on FR 15 lever-pressing in rats . Data are presented ae Mean t 3 .E .M . percent of control responding for each done of drug administered . S .E .M . e are not presented if less than the radius of the deta point . The number of animals in each q;oup is noted in parentheses . Under control conditions (saline i .c .v .) animals averaged 202 t 12 (Mesa t S .E .M ., n ~ 10 animals) reinforcers per 30 min session . Open figures (A-ß-endorphins o-morphine) shoo data from the initial dose-response experiments . Closed fiqgures show the data from subsequent infuaiona (see text) . Equations are given for the regression line for each drug determined by parallel line bioassay (20) . EDSOs (and 95ti confidence intervals) are given in the figure . Least squares linear regraasion analyses of log-transformed data revealed significant negative regressions between the dose of either ß-endorphin (F ~ 23 .36 for 1 and 8 df, P < .005) or morphine (F ~ 8 .38 for 1 and 6 df, P < .OS) and the rate of responding .
430
ß-Endorphin and Conditioned Behavior
Vol . 21, No . 7, 1977
11s can be seen in Figure 1, 5 .3, 14 and 29 nmoles of morphine produoed a dose-related suppression in fixed-ratio responding over the thirty minute ezperimental session . The ED50 of morphine, under these conditions is 14 .4 nmole . On a molar basis, ß-endorphin is 21 times more potent than morphine in auppreseinq fixed-ratio responding . Cochin and ICOrneteky (19) have reported that prior treatment pith morphine may inducé long lasting changes (tolerance) in the analgesic response of rata If such changes also occur following ß on subsequent exposure to morphine . endorphin, the dose-response relationships and hence the relative potencies of ß-endorphin and morphine may have been altered by repeated drug administration . However, this does not appear to be the case . The disruptive effect, on fixedratio responding, of the estimated ED50 of ß-endorphin (0 .7 nmole) and SD33 of morphine (8 .4 nmole) were not different than predicted, i .e . 47 t 14t and 33 t 13~ respectively (Fig . 1) . Since, under these experimental conditions, prior exposure to ß-endorphin or morphine did not alter the response of subjects to a second exposure to the same compound, within 2-3 weeks, it ae~e unlikely that ß-endorphin altered the response of rata to subsequent exposure to morphine and even more unlikely that a single exposure to morphine 5 months previously would have altered the response of the rats to ß-endorphin .
Rat L~7
Saline
N
m
Q{
O O v
ß-Endorphin (0 .7 nmoles)
Morphine (8.4 nmoles)
30 Minutes FIG . 2 Sample cumulative records for rat L-7 showing FR 15 responding immediately after i .c .v . infusion of 10 ul saline, 0 .7 nmole ß-endorphin or 8 .4 nmole morphine . Responding rate is reflected by the elope of the recording . Delivery of a reinforcer is indicated as a pip on the ascending record .
Vol . 21, No . 7, 1977
ß-Hndorphia sad Conditioned Behavior
931
Visual analysis of the behavioral sessions revealed that during the period in which the rats wars not lever-pressing thsy oitea engaged in behavior which varied with the dop of ß-enWorphin administered. Following the lowest dose (0 .3 nmoie) of ß-endorphin, the rats ware observed to engage is multiple boute of head and ear scratching . Scratching occurred bilatarally and was not related to the particular ventricle infused with peptide. At 1 .5 nmols, occasional wet-dog shakes appeared, in addition to the scratching . At 2 .9 nmola, acratchinq and considerably more wet-dog shaken (over 125 in one rat) occurred throughout the thirty ainute session. Scratching and wet-dog shakes were not seen lollowiag saline or morphine infusions . It should be pointed out that the amount of time spent scratching and/or shaking was only a fraction of the total time the rats ware not lever-pressing and could not account for the disruption in this behavior (except perhaps at the lowest dose) . Figure 2 shows examples of cumulative records of lever-pressing behavior alter injection of saline, ß-endorphin or morphine i .c .v . Although short bouts of scratching occurred after ß-endorphin but not alter morphine, the pattern of responding after both drugs looks quite similar. Pausing after delivery of the reinforcvrs ie increased, while running rates (respondinq during bursts or rune after postreinforcemant pauses) remain relatively constant and atippear identical to running rates during the saline session. . The highest dosaa of ß-endorphin and morphine produced an abrupt cessation of responding, typically immediately after delivery of a reinforcer . From these experiments it is seen that= (1) Centrally administered ß-endorphin and morphine suppress fixed-ratio operant behavior in a dose-dependant manner . (2) On a molar basis, ß-endorphin is 21 times more potent than morphine in its capacity to suppress this behavior . (3) Tolaraaoe to the behavioral suppressant action of either drug is not seen 2-3 weeks following an initial exposure . (4) Scratching and wet-dog shakes are seen only after ß-endorphin but do not entirely acwunt for the disruption in the behavior . To date, most investigations haw dealt with the similarities of ß-endorphin and morphine . However, in light of the differences between the two (catatonia (4,10), scratchin~ and wet-dog shakes seen only after ß-endorphin) future iavestigation to nocount for these differences is warranted. AçknoaledQ~nt This research was supported in part by USP~B Grant DA-00532 . References C. H. Li and D. Chunq, Froc . Natl . Aced . Sci. 73 1145-1148 (1976) . J. eläsiq and A. Hors, Nauayn-Schmiedeberq's Arch . Phaxmacol. 294 297-300 (1976) . 3. L. Graf, J. I. Szekely, A. Z . Roam, Z. Durai-1COVacs and S . Bajusz, Nature 263 240-241 (1976) . 4. Y .F. Sacquat and N . Marks, Science 194 632-635 (1976) . 5 .~. H. Loh, L. F . Tsenq, E . 9iei and C . H. Li, Pzoc . Natl . Aced . Sci . _73 2895-2898 (1976) . 6. H. Teschamachsr, J. Bläsiq and Vi . Bramer, Naunyn-Schmiedaberq's Aroh . Pharmacol . _294 293-295 (1976) . 7. J. M. van Ree, D. Deified, A. F . Bradbury, E. C . Hulme, D. C . Smyth and C. R. Snell, Nature 264 792-794 (1976) . 8. E. wei and H. Loh, Scieras _193 1262-1263 (1976) . 9 . L. F. Tsenq, H. H. Loh sad C. H. Li, Proc . Natl . Aced . Sci. _73 4187-4189 (1976) . F. E. Bloom, D. 8ega1, N. Longard sad R. Guilleman, Science _194 630-632 (1976) . 1. 2.
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R . T . Reueher and W . H . Morse, Ergeb . Physiol . _60 1-56 (1968) . T . Thampaon and J . J . Horen, Handbook of Operant Behavior , W . R . Honig and J . E . R. Staddon, Eds ., pp . 540-569, Prentice-Hall, Englewood Cliffs, N .J . (1977) . D . E . McMillan, Adv . Hiochem . Psychopharmacol . 8 345-359 (1973) . S . B . Sparbar, V . F . Gallert, L . Lichtblau and R. Eisenberg, Sympoai~an on Factors Affecting the Action of Narcotics , Milan, Italy, July 1976, M . W . Adler, L . Marsara and R . Samanin, Eds ., in press . J . E . Skinner, Neuroscience : A Laboratory Manual , pp . 195-237, W . H . Saun dera, Philadelphia, Pa . (1971) . S . B . Sparher, DaCI Carrier _2 1-5 (David Kopf Instruments, Tujunga, Calif ., 1975) . H . A . Tilson, R . H . Rech and S . B . Sparher, Pharmacol . Biochem . Behau . _3 385-392 (1975) . C . H . Li, 3 . Lemaire, D . Yamashiro and B . A . Doneen, Biochem . Biophys . Rea . C~un . _71 19-25 (1976) . J . Cochin and C . Rornetsky, J . Phas~acol . Exp . Ther . 145 1-10 (1964) . D . J . Finney, Statistical Methods in Hioloqical Assay, 2nd ed ., Hafner Publishing, New York, N .Y . (1964) .