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Peptide inhibition of morphine-induced dopaminergic supersensitivity
Life Sciences, Vol. 31, pp. 2287-2290 Printed in the U.S.A.-
Pergamon Press
PEPTIDE INHIBITION OF MORPHINE-INDUCED DOPAMINERGIC SUPERSENSITIVITY R.F. Ritzmann, l J.M. Lee I and J.Z. Fields 2 I U n i v e r s i t y of I l l i n o i s at the Medical Center Chicago, IL 60612 2Chicago Medical School North Chicago, IL 60064 (Received in final form June 14, 1982)
Summary I n j e c t i o n of the peptide cyclo(Leu-Gly) into rats p r i o r to chronic exposure to morphine, i n h i b i t s : I) the development of analgesic tolerance; 2) some signs of physical dependence; and, 3) morphineinduced increases in behavioral responses to dopamine agonists. Although there was no change in the t o t a l number of high a f f i n i t y s t r i a t a l dopamine receptors, chronic morphine treatment did increase the a f f i n i t y of the ligand at the receptor. The peptide blocked not only the a f f i n i t y change, but the increased behavioral response to apomorphine as w e l l . These behavioral changes c o r r e l a t e s i g n i f i cantly with the neurochemical changes in dopamine receptors f o l l o w ing chronic morphine treatment. Therefore, some of the pharmacological e f f o r t s of morphine may be mediated by changes in CNS dopamine receptors and that the peptides may act by i n h i b i t i n g these neurocilemical changes. Cyclo(Leu-Gly) (cLG), has been found to a l t e r the behavioral responses to several dopaminergic compounds (1,6). In a d d i t i o n , they have been shown to prevent the increased response to dopamine (DA) agonists which occurs as a r e s u l t of chronic morphine administration ( 2 , 3 ) , long-term haloperidol treatments (4) or chemical lesions of DA neurons with 6-OHDA (5). The neurochemical mechanism by which these peptides produce these e f f e c t s , however, is unknown. Peptide treatment did not a l t e r the a c t i v i t i e s of enzymes that synthesize or degrade DA in a way that could adequately account for t h e i r behavioral e f f e c t s . On the other hand, MIF produces an increase in DA agonist binding a f f i n i t y in v i t r o (8). Administration of cLG to rats produced an increase in apomorphine -Z , T ~ i,duced stereotypy (3), but did not a l t e r APO-induced hypothermia (4,5). However, cLG blocked the morphine-induced, increased behavioral responses to APO (stereetypy and hypothermia). In c o r r e l a t i n g DA receptor binding with behavior, i t was fcund that morphine, which increased stereotypy, also produced an i n crease in the s t r i a t a l DA receptor a f f i n i t y , but not in the t o t a l receptor density. This change in a f f i n i t y was blocked by the peptide. Injection of the peptide by i t s e l f did not produce a change in any antagonist binding parameters (3). While the above finding f o r antagonist binding could account f o r the e f fect of cLG on morphine-induced changes in the s t r i a t a l DA system, they are not consistent with the e f f e c t of the peptide alone. Therefore, in the present series of experiments, the e f f e c t of in vivo administration of the peptide was correlated with the binding of DA agonists. Since cLG increases APO-induced stereotypy ( s t ~ i a t a l DA receptors), but does not a f f e c t APO-induced hypethermia 0024-3205/82/202287-04503.00/0 Copyright (c) 1982 Pergamon Press Ltd.
2288
Morphine-induced Changes Modified
Vol. 31, No.s 20 & 21, 1982
(hypothalamic DA receptors), DA receptor agonist binding was studied in both brain regions in chronic morphine-treated and control rats. Methods Prior to use, male Wistar rats (Charles River, Wilmington, MA), weighing 250 ±25 g, were housed f o r at least seven days in rooms with controlled temperature (23 ° ± I°C) and l i g h t (0800 to 1800 hours). Food and water were available ad libitum, cLG was synthesized in our laboratory as described previously (15), dissolved in water and injected subcutaneously ( s . c . ) . Morphine was administered by s.c. implantation of s i l i c o n e p e l l e t s , each containing 65 mg of morphine (free base). Pellets were made as described by McGinity and Mehta (16). One group ( I ) of rats received an i n j e c t i o n of cLG (8 mg/kg), while another group ( I I ; controls) received only a vehicle i n j e c t i o n . Two hours l a t e r , half of each group received two morphine p e l l e t s (IA, I I A ) , half placebo p e l l e t s (IB, liB). On Day 2 of morphine treatment, three additional p e l l e t s were implanted in the four respective groups (see Table i ) . All p e l l e t s were removed 72 hours a f t e r the f i r s t implantation. Subsequent to p e l l e t removal (30 minutes), half of the rats from each group were injected with the morphine antagonist, naloxone (I0 mg/kg), and tested for physical dependence by monitoring body temperature, wet dog shakes, diarrhea and body weight, as described elsewhere (3). Animals not injected with naloxone were tested 24 hours a f t e r p e l l e t removal f o r behavioral responses to the DA agonist, APO (0.25 to 1.0 mg/kg), by monitoring the i n t e n s i t y of stereotypic behavior f o r 15 minutes a f t e r the i n j e c t i o n using the following rating scale: I = decreased locomotor behavior; 2 = normal movement; 3 = increased locomotor a c t i v i t y with rearing, bobbing and non-continuous s n i f f i n g ; 4 = stereotyped (continuous) s n i f f i n g ; 5 = l i c k i n g and gnawing (3). The EDen f o r APO to induce at least Type 4 stereotypic behavior was determined. To measure APO-induced hypothermia, core body temperature was determined j u s t p r i o r to i n j e c t i o n of APO and again 30 minutes l a t e r . Twenty-four hours a f t e r APO inj e c t i o n , rats were s a c r i f i c e d , s t r i a t a and hypothalami were removed and homogenized, and membranes containing the DA receptors were sedimented (1:100 d i l u tion) and resuspended. Displacement of 3H-spiroperidol (I00 pM) binding by DA was determined. Assay conditions f o r binding included: volume, 2 ml; b u f f e r , 50 mM Tris HCI, pH 7.4; time to equilibrium, 30 minutes; and, temperature, 37°C. Unlabelled DA solutions were constituted j u s t before use and added in the i n d i cated concentrations. Although s p e c i f i c DA receptor binding is usually defined as that f r a c t i o n of total 3H-spiroperidol binding that is i n h i b i t e d by d-(+)-butaclamol (IO-6M), we have shown (3) that there are two classes of sites with K~ app values of about 50 pM and 1,000 pM, respectively. Therefore, 3H-spiroperi~ dol was at I00 pM so that e s s e n t i a l l y only the higher a f f i n i t y (50 pM) s i t e is l a b e l l e d . Binding parameters at the lower a f f i n i t y s i t e do not appear to change consistently with changes in behavior under any of our drug treatments. Slope factors (Pseudo-Hill C o e f f i c i e n t s ) were determined using the quantity log [B/ B x-B] v. N log [ I ] formula as described elsewhere (11). Because these are ind i r e c t measurements of binding over a large concentration range of displacer (10 -12 - IO-4M), these slope factors are not true H i l l Coefficients. However, the extent to which the values d i f f e r from I indicates e i t h e r cooperativity or existence of multiple classes of sites ( i . e . , high and low a f f i n i t y s i t e s ) . Recent findings have shown the existence of DA i n h i b t i o n curves which are biphasi c , suggesting the p o s s i b i l i t y of two sites f o r DA (as opposed to 3H-spiroperidol) with apparent a f f i n i t i e s of 10-9 and IO-6M (11). Results Treatment of rats for three days with morphine resulted in a marked degree of physical dependence. This was exemplified by marked hypothermia in the
Vol. 31, No.s 20 & 21, 1982
Morphine-induced Changes Modified
2289
morohine-treated group (AT = -0.87 ± 0.17) compared to control rats (AT = 0.01 ± 0.20) (P < 0.05). As previously reported for mice (8), and more recently for rats (12), treatment with cLG p r i o r to morphine prevented the naloxone-induced hypothermia (AT = -0.20 ± 0.05). Peptide i n j e c t i o n into control rats did not a l t e r body temperature (AT = -0.08 ± 0.05). Other signs of morphine-induced physical dependence (weight loss, shakes, diarrhea) were not affected by the peptide (3). Twenty-four hours a f t e r p e l l e t removal, animals were tested for APO-inudced stereotypy. Morphine treatment results in a s h i f t to the l e f t of the dose-response curve for APO, as exemplified by a change in the ED from 0.75 mg/kg in the control rats to 0.42 mg/kg in the morphine-treated r a ~ (Table 1) (P < 0.05 by Fischer exact p r o b a b i l i t y t e s t ) . Giving peptide with morphine blocked the morphine-induced s h i f t to the l e f t , but only p a r t i a l l y (EDen = 0.56 mg/kg). The fact that administration of peptide alone produced a shi~% of the curve to the l e f t (0.52 mg/kg, P < 0.05), compared to controls, might account f o r the apparent i n a b i l i t y of the peptide to completely block the morphine-induced changes. In p a r a l l e l to these s h i f t s in the ED. 0 values for APO, the slope factors for DA i n h i b i t i o n of 3H-spiroperidol bindin~ co-varied with a c o r r e l a t i o n f a c t o r greater than 0.9 (log Probit p l o t ) , indicating that agonist binding may account for the behavior observed in all four groups (Table 1). TABLE i Effect of cLG on Morphine-lnduced Changes in Dopamine Systems Striatum
Drug Treatment
ED50 APO-Induced Stereotypy (mg/kg)
Hypothalamus
Slope Factor
0.5 mg/kg APO-Induced Hypothermia AT (°C)
Slope Factor
VEH/Placebo ( l i B )
0.75
0.85
-0.20 ± 0.25
0.34
VEH/Morphine (IIA)
0.42*
0.25
-0.85 ± 0.10"
0.16
Peptide/Placebo (IB)
0.56*
0.48
-0.20 ± 0.15
0.44
Peptide/Morphine (IA)
0.52*
0.49
-0.01 ± 0.i0
0.33
~2 = 0.990
y2 = 0.810
*P < 0.05, compared to controls (see t e x t f o r methods). The hypothalamic mediated temperature response to APO in the chronic morphine treated group, was an exaggerated response 30 minutes p o s t - i n j e c t i o n (0.5 mg APO). The mean body temperature f o r controls was AT 0.20 ± 0.25, while the morphine group was 0.85 ± 0 . I 0 ; this increased responsiveness to APO was blocked by cLG (AT 0.22 ± 0.15). In contrast to the s t r i a t a l system, the peptide alone did not potentiate the response to APO. Again, in p a r a l l e l to the increase in APO-induced hypothermia following chronic morphine administration, there was an a l t e r a t i o n in DA binding in the hypothalamus, r e f l e c t e d in a low slope f a c t o r (Table i) which was blocked by cLG ( c o r r e l a t i o n of body temperature change and slope factor: y2 = 0.810). The peptide, by i t s e l f , f a i l e d to increase the hypothalamic binding of DA which is consistent with i t s lack of e f f e c t on APOinduced hypothermia. I t is i n t e r e s t i n g to note that the peptide-only group act u a l l y showed a trend toward s u b - s e n s i t i v i t y to APO and a p a r a l l e l trend toward
2290
Morphine-induced Changes Modified
a decreased potency of DA f o r specific birding).
Vol. 31, No.s 20 & 21, 1982
3H-spiroperidol displacement by DA (only 20% of
Discussion The phenomenon that neuropeptides a f f e c t both receptor binding of neurotransmitters and, consequently, physiological or behavioral responses is not new (12). Although peptides could be a c t i r g e i t h e r as neuromodulators or as neurot r a n s m i t t e r s , t h i s controversy is beyond the scope of the presence discussion. Suffice i t to say that the mechanism by which morphine acts on the DA systems has been shown to be biphasic. Acute administration leads to an increase in DA turnover, whereas chronic morphine has been shown to decrease D# turnover (13, 14). I t has been postulated that i t is t h i s sustained decrease in DA turnover which leads to the exaggerated response to D~ agonists (7). The precise mechanisms of the actions of these peptides have eluded investigators f o r some time. The present study indicates that both behavioral and biochemical parameters are increased following morphine and that both are modified by p r i o r peptide t r e a t ment. Mereover, the mechanism of action of cLG more l i k e l y involves an a l t e r a tion in binding of agonists, rather than antagonists at the DA receptor. Acknowlcdgments Supported in part by grants from NIH (MH-33991, BRSG RR-5366, DA-3015). J.M.L. is a r e c i p i e n t of an NIH Pre-Doctoral Research Training Fellowship in the Pharmacological Sciences. References I. 2. 3. 4. 5. 6.
7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
N.P. PLOTINIKOFF and A.J. KASTIN, Arch. I n t . Pharmacodyn. Ther. 211 2!I-224 (1974). R.F. RITZMANri, R. WALTER, H.N. BHARGAVA and L.B. FLEXNER, Proc. Natl. Acad. S c i . , USA, 76 5997-5998 (1979). R.F. RITZMAN~, J.M. LEE and J.Z. FIELDS, Life Sci. 30 1573-1581 (1982). H.N. BHARGAVA and R.F. RITZMANN, Pharmacol. Biochem. & Behav. 13 633-636 (1980). R.F. RITZMANN and H.N. BHARGAVA, Life Sci. 27 2075-2080 (1980). J.P. HUIDOBRO-TORO, A. SCOTTI DE COROLIS and V.G. LONGO, Pharmac. Biochem. Behav. 2 105-109 (1974). M. ARY,-B. COX and P. LOMAX, J. Pharmacol. Exp. Ther. 200 271-276 (1977). S. CHIU, C.S. PAULOSE and R.K. MISHRA, Peptides 2 105-111 (1981). R.M. KOSTRZEWA, A.J. KASTIN and S.K. SOBRIAN, Ph%rmac. Biochem. & Behav. 3 1017-1023 (1975). R.M. KOSTRZEWA, H. FUKUSHIMA, C.T. HARSTON, K.W. PERRY, R.W. FULLER and A. J. KASTIN, Brain Res. Bull. 4 799-802 (1979). R.S. KENT, A. DE LEAN and R.J. LEFKOWITZ, Mol. Pharm. 17 14-23 (1980). E. COSTA, A. GUIDOTTI, I. HANBAUER, T. HEXUM, L. SAIANI, S. STINE and H. YANG, Fed. Proc. 40 160-164 (1981). B. COX, Characteristics and Function of Opioids, pp.2-13, E l s e v i e r , Lausanne, Switzerland (1978). J.H. JAFFE and S.K. SHARPLESS, Addictive States, American Association ~lervous and Mental Disorders, 46, pp. 226-246, Williams and Wilkins, Baltimore (1968). E. FISHER, Chem. Bet. 39 2893-2931 (1906). J.W. MC GINITY and C.S.---MEHTA, Pharmacol. Biochem. Behav. 9 705-708 (1978).
Pergamon Press
PEPTIDE INHIBITION OF MORPHINE-INDUCED DOPAMINERGIC SUPERSENSITIVITY R.F. Ritzmann, l J.M. Lee I and J.Z. Fields 2 I U n i v e r s i t y of I l l i n o i s at the Medical Center Chicago, IL 60612 2Chicago Medical School North Chicago, IL 60064 (Received in final form June 14, 1982)
Summary I n j e c t i o n of the peptide cyclo(Leu-Gly) into rats p r i o r to chronic exposure to morphine, i n h i b i t s : I) the development of analgesic tolerance; 2) some signs of physical dependence; and, 3) morphineinduced increases in behavioral responses to dopamine agonists. Although there was no change in the t o t a l number of high a f f i n i t y s t r i a t a l dopamine receptors, chronic morphine treatment did increase the a f f i n i t y of the ligand at the receptor. The peptide blocked not only the a f f i n i t y change, but the increased behavioral response to apomorphine as w e l l . These behavioral changes c o r r e l a t e s i g n i f i cantly with the neurochemical changes in dopamine receptors f o l l o w ing chronic morphine treatment. Therefore, some of the pharmacological e f f o r t s of morphine may be mediated by changes in CNS dopamine receptors and that the peptides may act by i n h i b i t i n g these neurocilemical changes. Cyclo(Leu-Gly) (cLG), has been found to a l t e r the behavioral responses to several dopaminergic compounds (1,6). In a d d i t i o n , they have been shown to prevent the increased response to dopamine (DA) agonists which occurs as a r e s u l t of chronic morphine administration ( 2 , 3 ) , long-term haloperidol treatments (4) or chemical lesions of DA neurons with 6-OHDA (5). The neurochemical mechanism by which these peptides produce these e f f e c t s , however, is unknown. Peptide treatment did not a l t e r the a c t i v i t i e s of enzymes that synthesize or degrade DA in a way that could adequately account for t h e i r behavioral e f f e c t s . On the other hand, MIF produces an increase in DA agonist binding a f f i n i t y in v i t r o (8). Administration of cLG to rats produced an increase in apomorphine -Z , T ~ i,duced stereotypy (3), but did not a l t e r APO-induced hypothermia (4,5). However, cLG blocked the morphine-induced, increased behavioral responses to APO (stereetypy and hypothermia). In c o r r e l a t i n g DA receptor binding with behavior, i t was fcund that morphine, which increased stereotypy, also produced an i n crease in the s t r i a t a l DA receptor a f f i n i t y , but not in the t o t a l receptor density. This change in a f f i n i t y was blocked by the peptide. Injection of the peptide by i t s e l f did not produce a change in any antagonist binding parameters (3). While the above finding f o r antagonist binding could account f o r the e f fect of cLG on morphine-induced changes in the s t r i a t a l DA system, they are not consistent with the e f f e c t of the peptide alone. Therefore, in the present series of experiments, the e f f e c t of in vivo administration of the peptide was correlated with the binding of DA agonists. Since cLG increases APO-induced stereotypy ( s t ~ i a t a l DA receptors), but does not a f f e c t APO-induced hypethermia 0024-3205/82/202287-04503.00/0 Copyright (c) 1982 Pergamon Press Ltd.
2288
Morphine-induced Changes Modified
Vol. 31, No.s 20 & 21, 1982
(hypothalamic DA receptors), DA receptor agonist binding was studied in both brain regions in chronic morphine-treated and control rats. Methods Prior to use, male Wistar rats (Charles River, Wilmington, MA), weighing 250 ±25 g, were housed f o r at least seven days in rooms with controlled temperature (23 ° ± I°C) and l i g h t (0800 to 1800 hours). Food and water were available ad libitum, cLG was synthesized in our laboratory as described previously (15), dissolved in water and injected subcutaneously ( s . c . ) . Morphine was administered by s.c. implantation of s i l i c o n e p e l l e t s , each containing 65 mg of morphine (free base). Pellets were made as described by McGinity and Mehta (16). One group ( I ) of rats received an i n j e c t i o n of cLG (8 mg/kg), while another group ( I I ; controls) received only a vehicle i n j e c t i o n . Two hours l a t e r , half of each group received two morphine p e l l e t s (IA, I I A ) , half placebo p e l l e t s (IB, liB). On Day 2 of morphine treatment, three additional p e l l e t s were implanted in the four respective groups (see Table i ) . All p e l l e t s were removed 72 hours a f t e r the f i r s t implantation. Subsequent to p e l l e t removal (30 minutes), half of the rats from each group were injected with the morphine antagonist, naloxone (I0 mg/kg), and tested for physical dependence by monitoring body temperature, wet dog shakes, diarrhea and body weight, as described elsewhere (3). Animals not injected with naloxone were tested 24 hours a f t e r p e l l e t removal f o r behavioral responses to the DA agonist, APO (0.25 to 1.0 mg/kg), by monitoring the i n t e n s i t y of stereotypic behavior f o r 15 minutes a f t e r the i n j e c t i o n using the following rating scale: I = decreased locomotor behavior; 2 = normal movement; 3 = increased locomotor a c t i v i t y with rearing, bobbing and non-continuous s n i f f i n g ; 4 = stereotyped (continuous) s n i f f i n g ; 5 = l i c k i n g and gnawing (3). The EDen f o r APO to induce at least Type 4 stereotypic behavior was determined. To measure APO-induced hypothermia, core body temperature was determined j u s t p r i o r to i n j e c t i o n of APO and again 30 minutes l a t e r . Twenty-four hours a f t e r APO inj e c t i o n , rats were s a c r i f i c e d , s t r i a t a and hypothalami were removed and homogenized, and membranes containing the DA receptors were sedimented (1:100 d i l u tion) and resuspended. Displacement of 3H-spiroperidol (I00 pM) binding by DA was determined. Assay conditions f o r binding included: volume, 2 ml; b u f f e r , 50 mM Tris HCI, pH 7.4; time to equilibrium, 30 minutes; and, temperature, 37°C. Unlabelled DA solutions were constituted j u s t before use and added in the i n d i cated concentrations. Although s p e c i f i c DA receptor binding is usually defined as that f r a c t i o n of total 3H-spiroperidol binding that is i n h i b i t e d by d-(+)-butaclamol (IO-6M), we have shown (3) that there are two classes of sites with K~ app values of about 50 pM and 1,000 pM, respectively. Therefore, 3H-spiroperi~ dol was at I00 pM so that e s s e n t i a l l y only the higher a f f i n i t y (50 pM) s i t e is l a b e l l e d . Binding parameters at the lower a f f i n i t y s i t e do not appear to change consistently with changes in behavior under any of our drug treatments. Slope factors (Pseudo-Hill C o e f f i c i e n t s ) were determined using the quantity log [B/ B x-B] v. N log [ I ] formula as described elsewhere (11). Because these are ind i r e c t measurements of binding over a large concentration range of displacer (10 -12 - IO-4M), these slope factors are not true H i l l Coefficients. However, the extent to which the values d i f f e r from I indicates e i t h e r cooperativity or existence of multiple classes of sites ( i . e . , high and low a f f i n i t y s i t e s ) . Recent findings have shown the existence of DA i n h i b t i o n curves which are biphasi c , suggesting the p o s s i b i l i t y of two sites f o r DA (as opposed to 3H-spiroperidol) with apparent a f f i n i t i e s of 10-9 and IO-6M (11). Results Treatment of rats for three days with morphine resulted in a marked degree of physical dependence. This was exemplified by marked hypothermia in the
Vol. 31, No.s 20 & 21, 1982
Morphine-induced Changes Modified
2289
morohine-treated group (AT = -0.87 ± 0.17) compared to control rats (AT = 0.01 ± 0.20) (P < 0.05). As previously reported for mice (8), and more recently for rats (12), treatment with cLG p r i o r to morphine prevented the naloxone-induced hypothermia (AT = -0.20 ± 0.05). Peptide i n j e c t i o n into control rats did not a l t e r body temperature (AT = -0.08 ± 0.05). Other signs of morphine-induced physical dependence (weight loss, shakes, diarrhea) were not affected by the peptide (3). Twenty-four hours a f t e r p e l l e t removal, animals were tested for APO-inudced stereotypy. Morphine treatment results in a s h i f t to the l e f t of the dose-response curve for APO, as exemplified by a change in the ED from 0.75 mg/kg in the control rats to 0.42 mg/kg in the morphine-treated r a ~ (Table 1) (P < 0.05 by Fischer exact p r o b a b i l i t y t e s t ) . Giving peptide with morphine blocked the morphine-induced s h i f t to the l e f t , but only p a r t i a l l y (EDen = 0.56 mg/kg). The fact that administration of peptide alone produced a shi~% of the curve to the l e f t (0.52 mg/kg, P < 0.05), compared to controls, might account f o r the apparent i n a b i l i t y of the peptide to completely block the morphine-induced changes. In p a r a l l e l to these s h i f t s in the ED. 0 values for APO, the slope factors for DA i n h i b i t i o n of 3H-spiroperidol bindin~ co-varied with a c o r r e l a t i o n f a c t o r greater than 0.9 (log Probit p l o t ) , indicating that agonist binding may account for the behavior observed in all four groups (Table 1). TABLE i Effect of cLG on Morphine-lnduced Changes in Dopamine Systems Striatum
Drug Treatment
ED50 APO-Induced Stereotypy (mg/kg)
Hypothalamus
Slope Factor
0.5 mg/kg APO-Induced Hypothermia AT (°C)
Slope Factor
VEH/Placebo ( l i B )
0.75
0.85
-0.20 ± 0.25
0.34
VEH/Morphine (IIA)
0.42*
0.25
-0.85 ± 0.10"
0.16
Peptide/Placebo (IB)
0.56*
0.48
-0.20 ± 0.15
0.44
Peptide/Morphine (IA)
0.52*
0.49
-0.01 ± 0.i0
0.33
~2 = 0.990
y2 = 0.810
*P < 0.05, compared to controls (see t e x t f o r methods). The hypothalamic mediated temperature response to APO in the chronic morphine treated group, was an exaggerated response 30 minutes p o s t - i n j e c t i o n (0.5 mg APO). The mean body temperature f o r controls was AT 0.20 ± 0.25, while the morphine group was 0.85 ± 0 . I 0 ; this increased responsiveness to APO was blocked by cLG (AT 0.22 ± 0.15). In contrast to the s t r i a t a l system, the peptide alone did not potentiate the response to APO. Again, in p a r a l l e l to the increase in APO-induced hypothermia following chronic morphine administration, there was an a l t e r a t i o n in DA binding in the hypothalamus, r e f l e c t e d in a low slope f a c t o r (Table i) which was blocked by cLG ( c o r r e l a t i o n of body temperature change and slope factor: y2 = 0.810). The peptide, by i t s e l f , f a i l e d to increase the hypothalamic binding of DA which is consistent with i t s lack of e f f e c t on APOinduced hypothermia. I t is i n t e r e s t i n g to note that the peptide-only group act u a l l y showed a trend toward s u b - s e n s i t i v i t y to APO and a p a r a l l e l trend toward
2290
Morphine-induced Changes Modified
a decreased potency of DA f o r specific birding).
Vol. 31, No.s 20 & 21, 1982
3H-spiroperidol displacement by DA (only 20% of
Discussion The phenomenon that neuropeptides a f f e c t both receptor binding of neurotransmitters and, consequently, physiological or behavioral responses is not new (12). Although peptides could be a c t i r g e i t h e r as neuromodulators or as neurot r a n s m i t t e r s , t h i s controversy is beyond the scope of the presence discussion. Suffice i t to say that the mechanism by which morphine acts on the DA systems has been shown to be biphasic. Acute administration leads to an increase in DA turnover, whereas chronic morphine has been shown to decrease D# turnover (13, 14). I t has been postulated that i t is t h i s sustained decrease in DA turnover which leads to the exaggerated response to D~ agonists (7). The precise mechanisms of the actions of these peptides have eluded investigators f o r some time. The present study indicates that both behavioral and biochemical parameters are increased following morphine and that both are modified by p r i o r peptide t r e a t ment. Mereover, the mechanism of action of cLG more l i k e l y involves an a l t e r a tion in binding of agonists, rather than antagonists at the DA receptor. Acknowlcdgments Supported in part by grants from NIH (MH-33991, BRSG RR-5366, DA-3015). J.M.L. is a r e c i p i e n t of an NIH Pre-Doctoral Research Training Fellowship in the Pharmacological Sciences. References I. 2. 3. 4. 5. 6.
7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
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