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Tolerance to morphine-induced mydriasis in the rat pupil
Life Sciences, Vol. 28, pp. 2469-2475 Printed in the U.S.A.
Pergamon Press
TOLERANCE TO MORPHINE-INDUCED MYDRIASIS IN THE RAT PUPIL Charles H. Adler, Michael Robin, and Martin W. Adler Temple University Medical School, Department of Pharmacology 3420 N. Broad Street, Phila., Pa. 19140 (Received in final form March 16, 1981) Su=~ar7 Despite the commonly held view that tolerance does not develop to the pupillary effects of narcotics, recent studies have demonstrated tolerance to heroin-induced miosis in humans and to morphine-induced mydriasis in the mouse. Previous studies in this laboratory have shown that morphine produces a dose-related mydriasis and fluctuation (large amplitude, irregular oscillations in pupil diameter) in the rat pupil; the present study was designed to determine if tolerance develops to these effects following subcutaneous implantation of morphine pellets. We found that there is, indeed, tolerance to the morphine-induced mydrlasis, but it is not complete. Furthermore, statistically significant tolerance does not develop to morphine-induced fluctuations in the rat pupil. Until only recently, it was generally accepted that tolerance did not develop to the pupillary effects of morphine. Although it has long been associated with most of morphine's effects (e.g. analgesia, respiratory depression), tolerance had never been noted to its pupillary or constipatory effects, giving rise to the notion that it only develops to the depressant, and not to the excitant, actions of morphine. Recently, however, it has been demonstrated that tolerance develops to morphine's constlpatory effects (I). Furthermore, using carefully controlled clinical studies, Tress et al (2) examined the development of tolerance to the miotic effect of heroin. They have shown that tolerance occurs in this system as well, and that it is both dose- and time-dependent. A second, and more recent, study demonstrated that significant tolerance develops to morphine's mydrlatic action on the mouse pupil (3). As part of a larger project on the effects of acutely and chronically administered narcotics in the rat, this laboratory has been studying the pupillary response for several years now. Having established time and dose relationships of morphlne-induced mydriasis in the rat (4), we sought to investigate the development of tolerance to the mydrlatlc effect of morphine using an Inexpensive infrared video-pupillometer (5). Since a naloxone-sensitive fluctuation of pupillary size is a concomitant of morphine-induced mydriasis in the rat (4), the present study was also designed to determine if tolerance develops to this effect. Methods Adult male, albino rats (Sprague-Dawley strain from Zivic-Miller Laboratories, Pittsburgh, Pa.) weighing 300 to 360 g were used. Animals were housed in groups of six upon delivery and maintained in our facilities for at least 0024-3205/81 /222469-07S02.00/0 Copyright (c) 1981 Pergamon Press Ltd.
2470
Tolerance to Morphine-Induced Mydriasis
Vol. 28, No. 22, 1981
one week prior to use. They were kept at 22 ± 2°C and 50 ± 10% relative humidity on a standard 24-hour light cycle (7 A.M. on, 7 P.M. off). Purina laboratory chow and water were provided ad libitum. The following protocol was followed for all animals tested. A naive rat was placed in a quiet, light-proof room illuminated by two, soft-whlte, 40w fluorescent bulbs covered with a red acetate filter. Following a 20-minute adaptation period, the rat was wrapped in a 30X35-cm cloth fastened by strips of adhesive tape. The rat remained wrapped, with only the head and tall exposed, for 20 minutes. The same adaptation and wrapping technique was performed on the second and third day for 30 and 45 minutes, respectively. In addition, the rat was placed on a platform wlth restraint straps over its body and a padded head clamp to reduce all movements. The restraint did not impede the rat's circulation or respiration. The three days of acclimation reduced struggling and movement to a minimum on the test day. Although a degree of stress is probably present, no stress ulcers were observed. On the fourth day (the test day), the rat was placed in the room for the initial 20-minute adaptation period, then wrapped and restrained in the usual manner. Ten minutes following restraint, an infrared light source was turned on and the pupil was brought into focus on a closed-clrcult T.V. camera and monitor (5). Pupil diameter was measured directly from the T.V. monitor every 30 seconds throughout a 15-minute control period. A subcutaneous injection of saline was then given, followed five minutes later by a subcutaneous injection of either saline or 32 mg/kg of morphine sulfate (Mallinckrodt), a dose previously reported to produce maximum mydriasis in the rat (4). Monitoring and recording resumed one minute following the second injection and continued for 60minutes, with measurements taken once every 30 seconds. At the conclusion of the test period, the rat was unwrapped and the lights were turned on. One hour later, it was anesthetizedwith ether. A mld-dorsal incision was made and either two placebo pellets, or two morphine pellets [75 mg each, manufactured according to the method of Gibson and Tingstad (6)] were enclosed in a nylen sac and placed subcutaneously. The incision was closed with wound clips, and the rats quickly recovered. On the fifth and sixth days, the pelleted animal was wrapped in the usual manner and left restrained for 45 minutes. On the seventh day, the pellets were removed by making a second, mid-dorsal incision (under ether anesthesia) and pulling out the nylon sac. The rat was given 4 hours to recover from the procedure and to allow the residual morphine to be metabolized. No signs of abstinence were apparent at this time, and pupillary size was in the control range. The same testing procedure as on the fourth day was then followed, with all rats being given 32 mg/kg of morphine as the second injection. Thus, the experimental design used two groups of animals: placebo-pellete4 and morphlne-pelleted. The placebo-pelleted rats (Group i) received (a) saline prior to pelleting, and (b) an injection of morphine following removal of the pellets. The morphlne-pelleted animals (Group 2) received an injection of morphine both (a) prior to pelleting, and (b) after the pellets were removed. Pupil size was measured before and after each treatment, each animal thereby serving as its own control. Comparison of Group 1/treatment b (hereafter referred to as Group ib) with Group 2/treatment a (i.e., Group 2a) could be used to evaluate the effect, if any, of the pelleting procedures themselves, while the degree of tolerance was assessed by comparing Group 2a and 2b. As previously mentioned, pupil diameter was measured from the T.V. monitor every 30 seconds throughout a 15-minute control and 60-minute test period. The pre-drug, control pupil size was taken as the average of the 30 control readings
Vol. 28, No. 22, 1981
Tolerance to Morphine-lnduced Mydriasis
2471
Raw data gathered during the test period was averaged over 5-minute epochs, yielding 12 test periods. To eliminate inter-animal differences in control pupil size, the parameter used to compare the effect of morphine was percent change in pupil size, defined as % change = TP - CP × i00 CP where TP is the average pupil diameter during a particular 5-minute, post-drug epoch and CP is the mean control pupil diameter, the average of 30 control readings. Finally, the percent change in pupil diameter was averaged for each group, both pre- and post-pelleting, and plotted against the 5-minute time intervals. A comparison of the maximum change in pupil size was also made using the actual raw data to find the percent change in pupil size. Thus, % change = maximum post-dru~ - maximum pre-drug diameter × I00 maximum pre-drug The percent change was calculated using the average maximum pupil diameter for eachgroup. The fluctuation index was calculated by summing the differences between consectuive diameter measurements and dividing by the number of observations. Thus, the fluctuation index is a measure of the rate of change in pupil size over a given period of time. Results The graph in Figure 1 shows the average percent change in pupil size over the 60-minute test period, each time period representing a 5-minute epoch. As can be seen, the dependent animals (Group 2b) showed a significant decrease (p < 0.001) in the average percent change in pupil diameter when compared to the pre-pelleted results (Group 2a). Thus, these animals had become tolerant. A comparison of animals receiving morphine prior to pelleting (2a) with rats receiving morphine only after implantation with placebo pellets (ib) fails to reveal any significant differences between the groups. Thus, the pelleting procedure had no effect on the results. Table i shows the percent change in pupillary diameter as calculated from the average maximum pupillary diameters for each group, regardless of time. There was significant (p < 0.05) tolerance to the mydriatic effect of morphine (120% vs. 80%), although the tolerance was not complete. Figure 2 is a histogram of the mean of the average percent change in pupil size for all groups of rats from time period 4 through 12, or from the 16th minute through the 60th minute of the test period. The first three time periods were not included to allow time for absorption of the drug. The histogram also clearly illustrates that tolerance developed (compare 2a to 2b), although the tolerance was not complete (la vs. 2b). Figure 3 is a histogram of the fluctuation index for all groups. As expected, morphine produced a marked increase in pupillary fluctuation (compare la to other groups). However, no tolerance developed to this effect of the narcotic (2a vs. 2b). Although the pelleting procedure per s__eeappears to have a small effect on the index (Ib vs. 2a), this difference was not statistically significant (p > 0.05) using a 2-tailed ~-test.
2472
Tolerance to Morphine-Induced Mydriasis
Vol. 28, No. 22, 1981
14o[ /
I
// /
•o
.= 80
o--o~'-"'~I=I"
/ ~Q
°loiA/ U .o/O I
I 2 C l~
/ 2
/
I
-°
I 4
I 6
Time
Period
FIG I Average percent change in pupil size over 60 min. (Refer to t e x t for definition of groups).
/ 8
l 10
I 12
Vol. 28, No. 22, 1981
Tolerance to Morphine-lnduced Mydriasis
2473
TABLE I Effect of Morphine Pellets on the Pupillary Effects of Morphine in Rats.
Averase Maximum Pupillary Diameter
P/_e-Pelletin8
Post-Pelletin 8
Placebo-Pelleted (n=4) 15-Min Control
1.6 +_ 0.2 mm
1.4 _+ 0.I mm
Post-injection
2.0 + 0.i mm a
3.3 + 0.2
Average Change*
25%
b
136%
Morphine-Pel!eted (n=5) 15-Min Control
1.5 + 0.i mm
1.5 + 0.I mm
Post-injection
3.3 + 0.2 mm b
2.8 + 0.2 mm b
Average Change*
120%
80%
* percent change of maximum pupillary diameter a saline b morphine (32 mg/kg)
Discussion The widespread belief that tolerance does not develop to the pupillary effects of narcotics was recently disproved by Tress et al (2), using heroin in humans, and by Adler et al (3) using morphine in mice. These findings and our interest in the study of the rat pupil led us to study the development of tolerance to morphine's mydriatic effects in rats. In the present study we decided to test the animals before making them dependent; thus, each animal became its own control. We found that significant tolerance did develop to morphine's mydriatic effect, as shown by analysis of the maximumpupil diameter before and after morphine administration, and of the average pupil diameters during the 60-minute test period as well. Our findings, therefore, provide evi~ dence in yet another species that tolerance develops to the pupillary effects of narcotics.
2474
Tolerance to Morphine-lnduced Mydriasis
_ 120 f
Vol. 28, No. 22, 1981
o.e
"~ o. 100
~'
=
0.5
~' iI80
~=o.,
i
E o.~
,(
20[ lalb
-2a2b
0.¢
la lb 2a 2b Group
Group FIG 2 Mean of the average percent change in pupil size from 16 through 60 min after drug injection. (Refer to text for definition of groups).
FIG 3 The fluctuation index is the sum of the differences between consecutive diameter measurements divided by the number of observations. (Refer to text for definition of groups).
It is important to note that the tolerance is not complete and the rats still show mydriatic responses to morphine. The apparent absence of tolerance development to the pupillary fluctuation is another indication that tolerance to morphine's pupillary effects differs from that associated with most other actions of the narcotic. Since both the mydriasis and fluctuation seen with the acute administration of morphine have been shown to be reversed by naloxone (4), tolerance might have been expected to develop to both effects. It should be noted, however, that the present experiment used the same 3-day period of exposure which has been found to result in marked tolerance to the analgesic and body temperature effects of morphine. Whether a complete tolerance would result from a more prolonged period of exposure to the narcotic is not known. Our results, in conjunction with the reports of Tress et al (2) and Adler et al (3), demonstrate that tolerance to the effect of morphine on pupil size occurs whether the initial response to morphine is mlosis (human) or mydrlasls (mouse, rat) and whether morphine produces behavioral excitation (mouse) or behavioral depression (human, rat).
Vol. 28, No. 22, 1981
Tolerance to Morphine-lnduced Mydriasis
2475
Acknowledgements This work was supported by Grant No. DA 00376 from the National Institute on Drug Abuse. The authors wish to thank Dr. W.E. Smith for preparing the morphine pellets and Ms. E.B. Geller for help throughout all phases of the study. Appreciation is also expressed to Mr. R. Berretta for preparing the figures and to Ms. C. Francis for typing the manuscript. References I.
2. 3. 4. 5. 6.
T.F. BURKS, G°A. CASTRO, and N.W. WEISBRODT, In: Opiates and Endogenous Opioid Peptides, H.W. Kosterlitz, ed., pp. 369-376, North Holland Publishing Co.,Amsterdam (1976). K.H. TRESS, A.A. EL-SOBKY, W. AHERNE, and E. PIALL, Br. J. Clin. Pharmacol. 5 299-303 (1978). C.H. ADLER, O. KEREN, and A.D. KORCZYN, J. Neural Transm. 48 43-47 (1980). H. KLEMFUSS, R.J. TALLARIDA, C.H. ADLER, and M.W. ADLER, J. Pharmacol. Exp. Ther. 208 91-95 (1979). R.B. MURRAY and M.H. LOUGHNANE, J. Neurosci. Methods, in press. R.D. GIBSON and J.E. TINGSTAD, J. Pharmac. Sci. 59 426-427 (1970).
Pergamon Press
TOLERANCE TO MORPHINE-INDUCED MYDRIASIS IN THE RAT PUPIL Charles H. Adler, Michael Robin, and Martin W. Adler Temple University Medical School, Department of Pharmacology 3420 N. Broad Street, Phila., Pa. 19140 (Received in final form March 16, 1981) Su=~ar7 Despite the commonly held view that tolerance does not develop to the pupillary effects of narcotics, recent studies have demonstrated tolerance to heroin-induced miosis in humans and to morphine-induced mydriasis in the mouse. Previous studies in this laboratory have shown that morphine produces a dose-related mydriasis and fluctuation (large amplitude, irregular oscillations in pupil diameter) in the rat pupil; the present study was designed to determine if tolerance develops to these effects following subcutaneous implantation of morphine pellets. We found that there is, indeed, tolerance to the morphine-induced mydrlasis, but it is not complete. Furthermore, statistically significant tolerance does not develop to morphine-induced fluctuations in the rat pupil. Until only recently, it was generally accepted that tolerance did not develop to the pupillary effects of morphine. Although it has long been associated with most of morphine's effects (e.g. analgesia, respiratory depression), tolerance had never been noted to its pupillary or constipatory effects, giving rise to the notion that it only develops to the depressant, and not to the excitant, actions of morphine. Recently, however, it has been demonstrated that tolerance develops to morphine's constlpatory effects (I). Furthermore, using carefully controlled clinical studies, Tress et al (2) examined the development of tolerance to the miotic effect of heroin. They have shown that tolerance occurs in this system as well, and that it is both dose- and time-dependent. A second, and more recent, study demonstrated that significant tolerance develops to morphine's mydrlatic action on the mouse pupil (3). As part of a larger project on the effects of acutely and chronically administered narcotics in the rat, this laboratory has been studying the pupillary response for several years now. Having established time and dose relationships of morphlne-induced mydriasis in the rat (4), we sought to investigate the development of tolerance to the mydrlatlc effect of morphine using an Inexpensive infrared video-pupillometer (5). Since a naloxone-sensitive fluctuation of pupillary size is a concomitant of morphine-induced mydriasis in the rat (4), the present study was also designed to determine if tolerance develops to this effect. Methods Adult male, albino rats (Sprague-Dawley strain from Zivic-Miller Laboratories, Pittsburgh, Pa.) weighing 300 to 360 g were used. Animals were housed in groups of six upon delivery and maintained in our facilities for at least 0024-3205/81 /222469-07S02.00/0 Copyright (c) 1981 Pergamon Press Ltd.
2470
Tolerance to Morphine-Induced Mydriasis
Vol. 28, No. 22, 1981
one week prior to use. They were kept at 22 ± 2°C and 50 ± 10% relative humidity on a standard 24-hour light cycle (7 A.M. on, 7 P.M. off). Purina laboratory chow and water were provided ad libitum. The following protocol was followed for all animals tested. A naive rat was placed in a quiet, light-proof room illuminated by two, soft-whlte, 40w fluorescent bulbs covered with a red acetate filter. Following a 20-minute adaptation period, the rat was wrapped in a 30X35-cm cloth fastened by strips of adhesive tape. The rat remained wrapped, with only the head and tall exposed, for 20 minutes. The same adaptation and wrapping technique was performed on the second and third day for 30 and 45 minutes, respectively. In addition, the rat was placed on a platform wlth restraint straps over its body and a padded head clamp to reduce all movements. The restraint did not impede the rat's circulation or respiration. The three days of acclimation reduced struggling and movement to a minimum on the test day. Although a degree of stress is probably present, no stress ulcers were observed. On the fourth day (the test day), the rat was placed in the room for the initial 20-minute adaptation period, then wrapped and restrained in the usual manner. Ten minutes following restraint, an infrared light source was turned on and the pupil was brought into focus on a closed-clrcult T.V. camera and monitor (5). Pupil diameter was measured directly from the T.V. monitor every 30 seconds throughout a 15-minute control period. A subcutaneous injection of saline was then given, followed five minutes later by a subcutaneous injection of either saline or 32 mg/kg of morphine sulfate (Mallinckrodt), a dose previously reported to produce maximum mydriasis in the rat (4). Monitoring and recording resumed one minute following the second injection and continued for 60minutes, with measurements taken once every 30 seconds. At the conclusion of the test period, the rat was unwrapped and the lights were turned on. One hour later, it was anesthetizedwith ether. A mld-dorsal incision was made and either two placebo pellets, or two morphine pellets [75 mg each, manufactured according to the method of Gibson and Tingstad (6)] were enclosed in a nylen sac and placed subcutaneously. The incision was closed with wound clips, and the rats quickly recovered. On the fifth and sixth days, the pelleted animal was wrapped in the usual manner and left restrained for 45 minutes. On the seventh day, the pellets were removed by making a second, mid-dorsal incision (under ether anesthesia) and pulling out the nylon sac. The rat was given 4 hours to recover from the procedure and to allow the residual morphine to be metabolized. No signs of abstinence were apparent at this time, and pupillary size was in the control range. The same testing procedure as on the fourth day was then followed, with all rats being given 32 mg/kg of morphine as the second injection. Thus, the experimental design used two groups of animals: placebo-pellete4 and morphlne-pelleted. The placebo-pelleted rats (Group i) received (a) saline prior to pelleting, and (b) an injection of morphine following removal of the pellets. The morphlne-pelleted animals (Group 2) received an injection of morphine both (a) prior to pelleting, and (b) after the pellets were removed. Pupil size was measured before and after each treatment, each animal thereby serving as its own control. Comparison of Group 1/treatment b (hereafter referred to as Group ib) with Group 2/treatment a (i.e., Group 2a) could be used to evaluate the effect, if any, of the pelleting procedures themselves, while the degree of tolerance was assessed by comparing Group 2a and 2b. As previously mentioned, pupil diameter was measured from the T.V. monitor every 30 seconds throughout a 15-minute control and 60-minute test period. The pre-drug, control pupil size was taken as the average of the 30 control readings
Vol. 28, No. 22, 1981
Tolerance to Morphine-lnduced Mydriasis
2471
Raw data gathered during the test period was averaged over 5-minute epochs, yielding 12 test periods. To eliminate inter-animal differences in control pupil size, the parameter used to compare the effect of morphine was percent change in pupil size, defined as % change = TP - CP × i00 CP where TP is the average pupil diameter during a particular 5-minute, post-drug epoch and CP is the mean control pupil diameter, the average of 30 control readings. Finally, the percent change in pupil diameter was averaged for each group, both pre- and post-pelleting, and plotted against the 5-minute time intervals. A comparison of the maximum change in pupil size was also made using the actual raw data to find the percent change in pupil size. Thus, % change = maximum post-dru~ - maximum pre-drug diameter × I00 maximum pre-drug The percent change was calculated using the average maximum pupil diameter for eachgroup. The fluctuation index was calculated by summing the differences between consectuive diameter measurements and dividing by the number of observations. Thus, the fluctuation index is a measure of the rate of change in pupil size over a given period of time. Results The graph in Figure 1 shows the average percent change in pupil size over the 60-minute test period, each time period representing a 5-minute epoch. As can be seen, the dependent animals (Group 2b) showed a significant decrease (p < 0.001) in the average percent change in pupil diameter when compared to the pre-pelleted results (Group 2a). Thus, these animals had become tolerant. A comparison of animals receiving morphine prior to pelleting (2a) with rats receiving morphine only after implantation with placebo pellets (ib) fails to reveal any significant differences between the groups. Thus, the pelleting procedure had no effect on the results. Table i shows the percent change in pupillary diameter as calculated from the average maximum pupillary diameters for each group, regardless of time. There was significant (p < 0.05) tolerance to the mydriatic effect of morphine (120% vs. 80%), although the tolerance was not complete. Figure 2 is a histogram of the mean of the average percent change in pupil size for all groups of rats from time period 4 through 12, or from the 16th minute through the 60th minute of the test period. The first three time periods were not included to allow time for absorption of the drug. The histogram also clearly illustrates that tolerance developed (compare 2a to 2b), although the tolerance was not complete (la vs. 2b). Figure 3 is a histogram of the fluctuation index for all groups. As expected, morphine produced a marked increase in pupillary fluctuation (compare la to other groups). However, no tolerance developed to this effect of the narcotic (2a vs. 2b). Although the pelleting procedure per s__eeappears to have a small effect on the index (Ib vs. 2a), this difference was not statistically significant (p > 0.05) using a 2-tailed ~-test.
2472
Tolerance to Morphine-Induced Mydriasis
Vol. 28, No. 22, 1981
14o[ /
I
// /
•o
.= 80
o--o~'-"'~I=I"
/ ~Q
°loiA/ U .o/O I
I 2 C l~
/ 2
/
I
-°
I 4
I 6
Time
Period
FIG I Average percent change in pupil size over 60 min. (Refer to t e x t for definition of groups).
/ 8
l 10
I 12
Vol. 28, No. 22, 1981
Tolerance to Morphine-lnduced Mydriasis
2473
TABLE I Effect of Morphine Pellets on the Pupillary Effects of Morphine in Rats.
Averase Maximum Pupillary Diameter
P/_e-Pelletin8
Post-Pelletin 8
Placebo-Pelleted (n=4) 15-Min Control
1.6 +_ 0.2 mm
1.4 _+ 0.I mm
Post-injection
2.0 + 0.i mm a
3.3 + 0.2
Average Change*
25%
b
136%
Morphine-Pel!eted (n=5) 15-Min Control
1.5 + 0.i mm
1.5 + 0.I mm
Post-injection
3.3 + 0.2 mm b
2.8 + 0.2 mm b
Average Change*
120%
80%
* percent change of maximum pupillary diameter a saline b morphine (32 mg/kg)
Discussion The widespread belief that tolerance does not develop to the pupillary effects of narcotics was recently disproved by Tress et al (2), using heroin in humans, and by Adler et al (3) using morphine in mice. These findings and our interest in the study of the rat pupil led us to study the development of tolerance to morphine's mydriatic effects in rats. In the present study we decided to test the animals before making them dependent; thus, each animal became its own control. We found that significant tolerance did develop to morphine's mydriatic effect, as shown by analysis of the maximumpupil diameter before and after morphine administration, and of the average pupil diameters during the 60-minute test period as well. Our findings, therefore, provide evi~ dence in yet another species that tolerance develops to the pupillary effects of narcotics.
2474
Tolerance to Morphine-lnduced Mydriasis
_ 120 f
Vol. 28, No. 22, 1981
o.e
"~ o. 100
~'
=
0.5
~' iI80
~=o.,
i
E o.~
,(
20[ lalb
-2a2b
0.¢
la lb 2a 2b Group
Group FIG 2 Mean of the average percent change in pupil size from 16 through 60 min after drug injection. (Refer to text for definition of groups).
FIG 3 The fluctuation index is the sum of the differences between consecutive diameter measurements divided by the number of observations. (Refer to text for definition of groups).
It is important to note that the tolerance is not complete and the rats still show mydriatic responses to morphine. The apparent absence of tolerance development to the pupillary fluctuation is another indication that tolerance to morphine's pupillary effects differs from that associated with most other actions of the narcotic. Since both the mydriasis and fluctuation seen with the acute administration of morphine have been shown to be reversed by naloxone (4), tolerance might have been expected to develop to both effects. It should be noted, however, that the present experiment used the same 3-day period of exposure which has been found to result in marked tolerance to the analgesic and body temperature effects of morphine. Whether a complete tolerance would result from a more prolonged period of exposure to the narcotic is not known. Our results, in conjunction with the reports of Tress et al (2) and Adler et al (3), demonstrate that tolerance to the effect of morphine on pupil size occurs whether the initial response to morphine is mlosis (human) or mydrlasls (mouse, rat) and whether morphine produces behavioral excitation (mouse) or behavioral depression (human, rat).
Vol. 28, No. 22, 1981
Tolerance to Morphine-lnduced Mydriasis
2475
Acknowledgements This work was supported by Grant No. DA 00376 from the National Institute on Drug Abuse. The authors wish to thank Dr. W.E. Smith for preparing the morphine pellets and Ms. E.B. Geller for help throughout all phases of the study. Appreciation is also expressed to Mr. R. Berretta for preparing the figures and to Ms. C. Francis for typing the manuscript. References I.
2. 3. 4. 5. 6.
T.F. BURKS, G°A. CASTRO, and N.W. WEISBRODT, In: Opiates and Endogenous Opioid Peptides, H.W. Kosterlitz, ed., pp. 369-376, North Holland Publishing Co.,Amsterdam (1976). K.H. TRESS, A.A. EL-SOBKY, W. AHERNE, and E. PIALL, Br. J. Clin. Pharmacol. 5 299-303 (1978). C.H. ADLER, O. KEREN, and A.D. KORCZYN, J. Neural Transm. 48 43-47 (1980). H. KLEMFUSS, R.J. TALLARIDA, C.H. ADLER, and M.W. ADLER, J. Pharmacol. Exp. Ther. 208 91-95 (1979). R.B. MURRAY and M.H. LOUGHNANE, J. Neurosci. Methods, in press. R.D. GIBSON and J.E. TINGSTAD, J. Pharmac. Sci. 59 426-427 (1970).