Experimental Section Gerontology 23: 165-173 (1977)
Exaggerated Response to (+)Amphetamine in Geriatric Gerbils1 Robert H. Tannen2 and Edward F. Domino Department of Pharmacology, University of Michigan, Ann Arbor, Mich.
Key Words. Gerbils • (+)Amphetamine • Locomotor activity • Thermoregulation Abstract. Young adult (6 -8 months) and old (greater than 24 months) gerbils were compared with respect to locomotor activity, body temperature and toxicity to (+)amphetaminc given intraperitoneally. Old gerbils showed greater locomotor activity to 5.6 mg/kg, greater hyperthermia to 10.0 mg/kg, and lower LDS0 (young adult 34.0 ± 7 mg/kg vs. old 17.6 ± 2.8 mg/kg) than young adults. These differences were not related to increased drug levels in whole blood or brain nor to diminished clearance of the drug from these sites. No age-related difference could be found in the ability to regulate body temperature in the face of extreme environmental temperatures nor could a difference be detected in the hypother mic response to the dopaminergic agonist apomorphine in doses of 5.6 mg/kg i.p.
Introduction Many investigators have observed an age-related resistance to the central nervous system stimulation produced by (+)amphetamine (1 ,4 , 12, 17, 19). Yet strong evidence exists that old animals are more sensitive than young ones to the production of physiological alterations mediated by the catecholamines epi nephrine and norepinephrine (6). Based on these reports, it was thought useful to obtain more direct evidence regarding a possible age-related alteration in the response to the actions of (+)amphetamine. This communication describes ex periments which demonstrate the existence of an age-related hypersensitivity to (+)amphetamine in the gerbil. 1 Supported in part by Psychopharmacology Research Fund. 2 Predoctoral fellow of USPHS training grant 76-GM-0860.
Downloaded by: Univ. of California Santa Barbara 128.111.121.42 - 3/6/2018 5:01:08 PM
Received: November 10, 1975.
Tannen /Domino
166
Materials and Methods
Downloaded by: Univ. of California Santa Barbara 128.111.121.42 - 3/6/2018 5:01:08 PM
Gerbils (Meriones unguiculatus) were obtained from Chick Line Company, Vineland, N.J. Young adult animals, aged 6 - 8 months, were from normal stock and weighed 64 + 1 g. Old animals, aged greater than 24 months, were retired breeders weighing 84 ± 2 g and were raised under conditions similar to those of the young adults. The designation ‘old’ is justi fied on the basis of mortality curves for the gerbil (18). Animals were housed in groups of 2-10 as received from the breeder. They were not regrouped, except to separate the breeding pairs. Cages were kept in a fluorcsccnt-lighted room maintained on a 12-hour light-dark cycle (7:00 a.m. to 7:00 p.m. light). Experiments were confined to the first 8 h of the light cycle. All experiments employed age- and sex-matched groups consisting, whenever possible, of the same number of males and females. It was necessary to use animals more than once because of the limited number of geriatric animals available. When this was done, 1-2 months elapsed between exposure to any experimental situation and the same number of young adult animals were reused as old ones. All drug dosages were calculated as base and administered intraperitoneally as salts; (+)amphetamine sulfate was obtained from Cal Biochem, apomorphine HC1 was obtained from Merck Chemicals, and heparin was obtained from Nutritional Biochemicals. The apomorphine was freed of its oxidized form by washing the powder with three 2-ml portions of diethyl ether in a ground glass homogenizer operated by hand. The crystals were air dried and used immediately. Locomotor activity was measured by placing single gerbils into circular metal chambers 245 cm in diameter and 85 cm high and each fitted with a pair of photo cells and focused light beams arranged perpendicularly to each other. The photo cells were wired to digital counters in such a way that whenever a beam of light was interrupted a single activity count was registered. The activity chambers were placed in a sound-resistant box in which the air temperature was maintained between 21 and 24 °C. Blood and brain levels of (+)amphetamine were determined using a modification of Brodie’s methyl orange technique (2, 3) with the following sample preparation: animals were sacrificed by decapitation and exsanguinated. Blood was collected in heparinized graduated test tubes through heparinized glass funnels. The glassware was prepared by rinsing with a 100 mg% sodium heparin solution and allowing it to dry. As an additional precaution against coagulation, a few small crystals of heparin were placed in the test tubes before collecting blood. Whole blood volumes were recorded. Then 1 (V HC1 was added to a total volume of 15 ml. Brains were removed from each animal immediately after collecting blood, weighed, and then homogenized in a motor driven ground glass homogenizer in 15 ml 1 N HC1. Both blood and brain samples were then heated 15 min over boiling water, centri fuged (10 min at l,000g) and the supernatant employed in a diethyl ether extraction as indicated in the Axelrod method. It was found that heating the sample reduced the blank values and allowed measurement too 0.5 ug of (+)amphetamine per sample. Temperature studies were performed with a remote telethcrmometcr (Yellow Springs Instrument Company, Yellow Springs, Ohio; model 41 TA) employing a thermistor rectal probe (Yellow Springs Instrument model No. 401). Experiments with (+)amphetamine and apomorphine were performed with individually housed animals at room temperature, 27 °C. Measurement of core temperature response to extreme environmental temperatures were performed in temperature-controlled rooms maintained at either 1 -2 or 42-46 °C. All animals were housed individually and left unrestrained when temperature was not being recorded. The Student’s t test for group comparisons was used to determine the significance of results.
(+)Amphetamine in Geriatric Gerbils
167
Log dose (♦) amphetamine, mg/kg
Fig. I. Locomotor response to (+)amphetaminc. Mean locomotor activity counts over an observation period of 180 min are plotted as a function of log dose (+)amphetamine. Each point represents 8-11 animals. Significance is calculated by group comparison t test: * p < 0.05.
Locomotor A ctivity Old and young adult gerbils were placed singly into activity chambers to determine whether age is a factor altering locomotor response to ^ a m p h e t amine. In both age groups a dose related increase in locomotor activity was noted with a maximum response occurring at 5.6 mg/kg (fig. 1). At higher doses, activity was reduced. Old gerbils showed a significantly greater response than young adults when doses of 3.2 mg/kg or higher were given. Some deaths oc curred among the old gerbils receiving 10.0 mg/kg whereas none of the young adult animals receiving this dose died.
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Results
Tannen /Domino
168
A mphetamine Hyperthermia A characteristic response to (+)amphetamine is a dose-related hyperthermia. Since this action of (+)amphetamine has been linked with its lethal effects (14), a dose which caused death among the old gerbils, 10.0 mg/kg, was chosen to compare the rectal temperature response in young adult and old animals. As noted in figure 2, a diphasic hyperthermic-hypothermic response was observed in all animals. A significantly greater hyperthermic response to (+)amphetamine was noted among the old gerbils. Deaths among both old and young adults were noted to occur during the hypothermic phase of the response. Inasmuch as deaths were observed among both age groups, it became im portant to determine if a definite increased toxicity to (+)amphetamine existed
Downloaded by: Univ. of California Santa Barbara 128.111.121.42 - 3/6/2018 5:01:08 PM
Fig. 2. Temperature time course after (+)amphetaminc. Rectal temperature is plotted as the change from the average of three measurements during a control period of 30 min prior to administering 10.0 mg/kg (+)amphctamine i.p. to both young adult and old gerbils. Significance by group comparison t test: * p < 0.05; ** p < 0.01.
169
(+)Amphetamine in Geriatric Gerbils Table I. LDS0 of (+)amphetamine in young adult and old gerbils Dose of (+)amphctamine S 04 mg/kg
5.6 10.0 12.5 15.0 20.0 30.0 LDS0 ± SE1
Mortality after 4 h, % young adult (n)
old (n)
0(4) 0(4) 0(4) 17(6) 50 (4) 75 (4) 34.0 ± 7.0 mg/kg2
0(4) 25 (4) 0(4) 34 (6) 50 (4) 100 (4) 17.6 ± 2.8 mg/kg
1 Calculated by the method of Miller and Tainter (16). 2 Significantly different from that for old animals (p < 0.001).
Time, min
Fig. 3. Drug levels in blood and brain after (+)amphctamine. Blood and brain levels of (+)amphetaminc at various times after giving 5.6 mg/kg i.p. to young adult and old gerbils. Each point represents the mean ± SE for four animals. No significant difference was found by the group comparison t test.
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in older animals. Accordingly, increasing doses of the drug were given intraperitoneally to groups of animals of both ages, and mortality after 4 h was recorded (table I). The LDS0 ± SE was calculated by the method of Miller and Tainter (16) and found to be 34.0 ± 7.0 mg/kg for young adults and 17.6 ± 2.8 mg/kg for old gerbils. This difference was highly significant (p < 0.001).
170
Fig. 4a, b. Temperature regulation in hot or cold environment. Rectal temperature is plotted as the change from the average of three measurements during a control period of 30 min prior to exposure of young adult and old gerbils to the test environment. Environ mental temperatures are indicated along the base of each figure. Each point represents the average change ± SE of five animals. No significant difference was found by the group comparison t test.
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Tannen ¡Domino
(+)Amphetamine in Geriatric Gerbils
171
Tissue Levels o f Amphetamine If kidney and/or liver function deteriorate in old gerbils or if there is a change in the permeability of the blood brain barrier with age, such changes would be reflected by drug levels in the blood and brain. In experiments mea suring blood and brain levies of amphetamine at various times after 5.6 mg/kg, a dose which produced significantly greater locomotor stimulation in old gerbils than in young adults, no significant difference in drug levels could be found between the age groups (fig. 3). Thermal Regulation It has been suggested that the ability to regulate body temperature is diminished in old age (10, 11, 20). The exaggerated hyperthermic response to (+)amphetamine might be explained if this were true of old gerbils as well. To test this possibility, groups of young adult and old gerbils were placed individual ly in either a very hot (42—46 °C) or very cold (1—2 °C) environment and their
Tim e, min
Downloaded by: Univ. of California Santa Barbara 128.111.121.42 - 3/6/2018 5:01:08 PM
Fig. 5. Temperature time course after apomorphine. Rectal temperature is plotted as the change from the average of three measurements during a control period of 30 min prior to administering 5.6 mg/kg i.p. apomorphine to both young adult and old gerbils. Each point represents the average change ± SE of five animals. No significant difference was found by the group comparison t test.
Tannen ¡Domino
172
rectal temperatures were monitored during and for some time after the environ mental challenge (fig. 4a, b). No significant difference between the two groups could be detected at any time. However, a peculiar temperature fluctuation could be observed among the old animals placed in a cold room. Apomorphine Hypothermia Dopamine may be involved in the central nervous system regulation of temperature (9, 13, 15). It has been suggested that (+)amphetamine may pro duce its hyperthermic effects via a dopaminergic mechanism (8). To test the possibility that a dopaminergic mechanism might be involved in the exaggerated hyperthermic response to (+)amphetamine in old gerbils, both young adult and old animals were given 5.6 mg/kg of apomorphine, a dopaminergic agonist, and their rectal temperature monitored. No significant difference between the two age groups could be found, although a marked hypothermia was produced along with behavioral excitation (fig. 5).
The experiments presented here demonstrate that old gerbils are more sensi tive than young adults to (+)amphetamine. Old animals show greater locomotor activity, greater hyperthermia and greater toxicity than young adults receiving comparable doses of the drug. It has been further shown that this age-related difference is not due to the amount of the drug which ultimately gets into the blood or brain or to the length of time it remains at these sites. No age-related difference could be found in the ability to regulate body temperature in the face of extreme environmental temperatures nor could a difference be detected in the hypothermic response to the dopaminergic agonist, apomorphine. These observa tions suggest that an inability to regulate temperature is not the cause of amphetamine hypersensitivity in the old gerbil and neither is increased sensi tivity of dopaminergic receptors the probable cause. Since an age-related hyper sensitivity was observed, it is possible that the direct actions of (+)amphetamine are different in old gerbils than in young adults. The findings of this investigation conflict with evidence which has been obtained supporting a resistance to (+)amphetamine stimulation (1 ,4 , 12, 17, 19). This apparent contradiction may ultimately be explained by species differ ences, since such variability is suggested with regard to the peripheral (7) or central (8) mediation of (+)amphetamine hyperthermia. The constellation of exaggerated responses to (+)amphetamine in old gerbils may prove to be interrelated in that higher ambient temperatures are associated with greater locomotor activity (5) and greater toxicity (14) in response to (+)amphetamine. To study this possibility, it would be necessary to dissociate the various components of the pharmacology of (+)amphetamine.
Downloaded by: Univ. of California Santa Barbara 128.111.121.42 - 3/6/2018 5:01:08 PM
Discussion
(+)Amphetamine in Geriatric Gcrbils
173
References
Dr. E.F. Domino, Department of Pharmacology, University of Michigan, Ann Arbor, MI 48109 (USA)
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1 Arnett, J.H. and Harris, S.E.: Effects of small doses of amphetamine sulfate upon aged. Geriatrics 3: 84-88(1948). 2 Axelrod, J.: Studies on sympathomimetic amines. II. The biotransformation and physiologic disposition of d-amphetaminc, d-phydroxy-amphetamine and d-methamphetamine. J. Pharmac. exp. Ther. 110: 315-326 (1954). 3 Brodie, B.B.: Udenfriend, S., and Dill, W.: The estimation of basic organic compounds in biological material. V. Estimation. J. biol. Chem. 168: 335-339 (1947). 4 Caveness, W.: Benzedrine sulfate in elderly people. N.Y. J. Med. 47: 1003-1005 (1947). 5 Chance, M.R.A.: Aggregation as a factor influencing the toxicity of sympathomimetic amines in mice. J. Pharmac. exp. Ther. 87: 214-219 (1946). 6 Frolkis, V.V.; Bezrukov, V.V.; Bogatskaya, N.S.; Verkhratsky, N.S.; Zamostian, P.; Shevtchuk, V.G., and Shtchegoleva, I. V.: Catecholamines in the metabolism and func tions regulation in aging. Gerontologia 16: 129-140 (1970). 7 Gessa, G.L.; Clay, G.A., and Brodie, B.B.: Evidence that hyperthermia produced by d-amphetamine is caused by a peripheral action of the drug. Life Sci. 8: 135-141 (1969). 8 Hill, H. and Horita, A.: Amphetamine induced hyperthermia in rabbits. Pharmacologist 12: 197 (1970). 9 Horita, A. and Hill, H.F.: Hallucinogens, amphetamine and temperature regulation. The pharmacology of thermoregulation, pp. 417-431 (Karger, Basel 1972). 10 Krag, C.L. and Kountz, W.B.: Stability of body functions in the aged. I. Effect of exposure of the body to cold. J. Geront. 5: 227-235 (1950). 11 Krag, C.L. and Kountz, W.B.: Stability of body function in the aged. II. Effects of exposure of the body to heat. J. Geront. 7: 61 -7 0 (1952). 12 Krugman, A.D.; Ross, S.; Vicino, F.L., and Clyde, D.J.: A research note. Effects of d-amphetaminc and meprobamate on problem solving and mood of aged subjects. J. Geront. 15: 419-420(1960). 13 Kruk, Z.L.: The effect of drugs acting on dopamine receptors on the body temperature of the rat. Life Sci. 11: 845-850 (1972). 14 Mantegazza, P.: Kabir, M.; Naimzada, M., and Riva, M.: Effects of propranolol on some activities of amphetamine. Eur. J. Pharmacol. 4: 25-30 (1968). 15 Matsumoto, C. and Griffin, W.: Antagonism of (+)amphetamine induced hyperthermia in rats by pimozide. J. Pharm. Pharmac. 23: 710 (1971). 16 Miller, L.C. and Tainter, M.L.: Estimation of the EDS0 and its error by means of logarithmic-probit graph paper. Proc. Soc. exp. Biol. Med. 57: 261-264 (1944). 17 Nathanson, M.H.: The central action of beta-amino propylbenzene (benzedrine). J. Am. mod. Ass. 108: 528-531 (1937). 18 Troup, G.M.; Smith, G.S., and Wolford, R.L.: Life span, chronologic disease patterns, and age related changes in relative spleen weights for the Mongolian gerbil (Meriones ungiuculatus). Exp. Gerontol. 4: 139-143 (1969). 19 Verzar, F.: The age of the individual as one of the parameters of pharmacological action. Acta physiol, hung. 19: 313-318 (1961). 20 Wagner, J.A.; Robinson, S., and Maurio, R.B.: Age and temperature regulation of humans in neutral and cold environments. J. appl. Physiol. 37: 562-565 (1974).
Exaggerated Response to (+)Amphetamine in Geriatric Gerbils1 Robert H. Tannen2 and Edward F. Domino Department of Pharmacology, University of Michigan, Ann Arbor, Mich.
Key Words. Gerbils • (+)Amphetamine • Locomotor activity • Thermoregulation Abstract. Young adult (6 -8 months) and old (greater than 24 months) gerbils were compared with respect to locomotor activity, body temperature and toxicity to (+)amphetaminc given intraperitoneally. Old gerbils showed greater locomotor activity to 5.6 mg/kg, greater hyperthermia to 10.0 mg/kg, and lower LDS0 (young adult 34.0 ± 7 mg/kg vs. old 17.6 ± 2.8 mg/kg) than young adults. These differences were not related to increased drug levels in whole blood or brain nor to diminished clearance of the drug from these sites. No age-related difference could be found in the ability to regulate body temperature in the face of extreme environmental temperatures nor could a difference be detected in the hypother mic response to the dopaminergic agonist apomorphine in doses of 5.6 mg/kg i.p.
Introduction Many investigators have observed an age-related resistance to the central nervous system stimulation produced by (+)amphetamine (1 ,4 , 12, 17, 19). Yet strong evidence exists that old animals are more sensitive than young ones to the production of physiological alterations mediated by the catecholamines epi nephrine and norepinephrine (6). Based on these reports, it was thought useful to obtain more direct evidence regarding a possible age-related alteration in the response to the actions of (+)amphetamine. This communication describes ex periments which demonstrate the existence of an age-related hypersensitivity to (+)amphetamine in the gerbil. 1 Supported in part by Psychopharmacology Research Fund. 2 Predoctoral fellow of USPHS training grant 76-GM-0860.
Downloaded by: Univ. of California Santa Barbara 128.111.121.42 - 3/6/2018 5:01:08 PM
Received: November 10, 1975.
Tannen /Domino
166
Materials and Methods
Downloaded by: Univ. of California Santa Barbara 128.111.121.42 - 3/6/2018 5:01:08 PM
Gerbils (Meriones unguiculatus) were obtained from Chick Line Company, Vineland, N.J. Young adult animals, aged 6 - 8 months, were from normal stock and weighed 64 + 1 g. Old animals, aged greater than 24 months, were retired breeders weighing 84 ± 2 g and were raised under conditions similar to those of the young adults. The designation ‘old’ is justi fied on the basis of mortality curves for the gerbil (18). Animals were housed in groups of 2-10 as received from the breeder. They were not regrouped, except to separate the breeding pairs. Cages were kept in a fluorcsccnt-lighted room maintained on a 12-hour light-dark cycle (7:00 a.m. to 7:00 p.m. light). Experiments were confined to the first 8 h of the light cycle. All experiments employed age- and sex-matched groups consisting, whenever possible, of the same number of males and females. It was necessary to use animals more than once because of the limited number of geriatric animals available. When this was done, 1-2 months elapsed between exposure to any experimental situation and the same number of young adult animals were reused as old ones. All drug dosages were calculated as base and administered intraperitoneally as salts; (+)amphetamine sulfate was obtained from Cal Biochem, apomorphine HC1 was obtained from Merck Chemicals, and heparin was obtained from Nutritional Biochemicals. The apomorphine was freed of its oxidized form by washing the powder with three 2-ml portions of diethyl ether in a ground glass homogenizer operated by hand. The crystals were air dried and used immediately. Locomotor activity was measured by placing single gerbils into circular metal chambers 245 cm in diameter and 85 cm high and each fitted with a pair of photo cells and focused light beams arranged perpendicularly to each other. The photo cells were wired to digital counters in such a way that whenever a beam of light was interrupted a single activity count was registered. The activity chambers were placed in a sound-resistant box in which the air temperature was maintained between 21 and 24 °C. Blood and brain levels of (+)amphetamine were determined using a modification of Brodie’s methyl orange technique (2, 3) with the following sample preparation: animals were sacrificed by decapitation and exsanguinated. Blood was collected in heparinized graduated test tubes through heparinized glass funnels. The glassware was prepared by rinsing with a 100 mg% sodium heparin solution and allowing it to dry. As an additional precaution against coagulation, a few small crystals of heparin were placed in the test tubes before collecting blood. Whole blood volumes were recorded. Then 1 (V HC1 was added to a total volume of 15 ml. Brains were removed from each animal immediately after collecting blood, weighed, and then homogenized in a motor driven ground glass homogenizer in 15 ml 1 N HC1. Both blood and brain samples were then heated 15 min over boiling water, centri fuged (10 min at l,000g) and the supernatant employed in a diethyl ether extraction as indicated in the Axelrod method. It was found that heating the sample reduced the blank values and allowed measurement too 0.5 ug of (+)amphetamine per sample. Temperature studies were performed with a remote telethcrmometcr (Yellow Springs Instrument Company, Yellow Springs, Ohio; model 41 TA) employing a thermistor rectal probe (Yellow Springs Instrument model No. 401). Experiments with (+)amphetamine and apomorphine were performed with individually housed animals at room temperature, 27 °C. Measurement of core temperature response to extreme environmental temperatures were performed in temperature-controlled rooms maintained at either 1 -2 or 42-46 °C. All animals were housed individually and left unrestrained when temperature was not being recorded. The Student’s t test for group comparisons was used to determine the significance of results.
(+)Amphetamine in Geriatric Gerbils
167
Log dose (♦) amphetamine, mg/kg
Fig. I. Locomotor response to (+)amphetaminc. Mean locomotor activity counts over an observation period of 180 min are plotted as a function of log dose (+)amphetamine. Each point represents 8-11 animals. Significance is calculated by group comparison t test: * p < 0.05.
Locomotor A ctivity Old and young adult gerbils were placed singly into activity chambers to determine whether age is a factor altering locomotor response to ^ a m p h e t amine. In both age groups a dose related increase in locomotor activity was noted with a maximum response occurring at 5.6 mg/kg (fig. 1). At higher doses, activity was reduced. Old gerbils showed a significantly greater response than young adults when doses of 3.2 mg/kg or higher were given. Some deaths oc curred among the old gerbils receiving 10.0 mg/kg whereas none of the young adult animals receiving this dose died.
Downloaded by: Univ. of California Santa Barbara 128.111.121.42 - 3/6/2018 5:01:08 PM
Results
Tannen /Domino
168
A mphetamine Hyperthermia A characteristic response to (+)amphetamine is a dose-related hyperthermia. Since this action of (+)amphetamine has been linked with its lethal effects (14), a dose which caused death among the old gerbils, 10.0 mg/kg, was chosen to compare the rectal temperature response in young adult and old animals. As noted in figure 2, a diphasic hyperthermic-hypothermic response was observed in all animals. A significantly greater hyperthermic response to (+)amphetamine was noted among the old gerbils. Deaths among both old and young adults were noted to occur during the hypothermic phase of the response. Inasmuch as deaths were observed among both age groups, it became im portant to determine if a definite increased toxicity to (+)amphetamine existed
Downloaded by: Univ. of California Santa Barbara 128.111.121.42 - 3/6/2018 5:01:08 PM
Fig. 2. Temperature time course after (+)amphetaminc. Rectal temperature is plotted as the change from the average of three measurements during a control period of 30 min prior to administering 10.0 mg/kg (+)amphctamine i.p. to both young adult and old gerbils. Significance by group comparison t test: * p < 0.05; ** p < 0.01.
169
(+)Amphetamine in Geriatric Gerbils Table I. LDS0 of (+)amphetamine in young adult and old gerbils Dose of (+)amphctamine S 04 mg/kg
5.6 10.0 12.5 15.0 20.0 30.0 LDS0 ± SE1
Mortality after 4 h, % young adult (n)
old (n)
0(4) 0(4) 0(4) 17(6) 50 (4) 75 (4) 34.0 ± 7.0 mg/kg2
0(4) 25 (4) 0(4) 34 (6) 50 (4) 100 (4) 17.6 ± 2.8 mg/kg
1 Calculated by the method of Miller and Tainter (16). 2 Significantly different from that for old animals (p < 0.001).
Time, min
Fig. 3. Drug levels in blood and brain after (+)amphctamine. Blood and brain levels of (+)amphetaminc at various times after giving 5.6 mg/kg i.p. to young adult and old gerbils. Each point represents the mean ± SE for four animals. No significant difference was found by the group comparison t test.
Downloaded by: Univ. of California Santa Barbara 128.111.121.42 - 3/6/2018 5:01:08 PM
in older animals. Accordingly, increasing doses of the drug were given intraperitoneally to groups of animals of both ages, and mortality after 4 h was recorded (table I). The LDS0 ± SE was calculated by the method of Miller and Tainter (16) and found to be 34.0 ± 7.0 mg/kg for young adults and 17.6 ± 2.8 mg/kg for old gerbils. This difference was highly significant (p < 0.001).
170
Fig. 4a, b. Temperature regulation in hot or cold environment. Rectal temperature is plotted as the change from the average of three measurements during a control period of 30 min prior to exposure of young adult and old gerbils to the test environment. Environ mental temperatures are indicated along the base of each figure. Each point represents the average change ± SE of five animals. No significant difference was found by the group comparison t test.
Downloaded by: Univ. of California Santa Barbara 128.111.121.42 - 3/6/2018 5:01:08 PM
Tannen ¡Domino
(+)Amphetamine in Geriatric Gerbils
171
Tissue Levels o f Amphetamine If kidney and/or liver function deteriorate in old gerbils or if there is a change in the permeability of the blood brain barrier with age, such changes would be reflected by drug levels in the blood and brain. In experiments mea suring blood and brain levies of amphetamine at various times after 5.6 mg/kg, a dose which produced significantly greater locomotor stimulation in old gerbils than in young adults, no significant difference in drug levels could be found between the age groups (fig. 3). Thermal Regulation It has been suggested that the ability to regulate body temperature is diminished in old age (10, 11, 20). The exaggerated hyperthermic response to (+)amphetamine might be explained if this were true of old gerbils as well. To test this possibility, groups of young adult and old gerbils were placed individual ly in either a very hot (42—46 °C) or very cold (1—2 °C) environment and their
Tim e, min
Downloaded by: Univ. of California Santa Barbara 128.111.121.42 - 3/6/2018 5:01:08 PM
Fig. 5. Temperature time course after apomorphine. Rectal temperature is plotted as the change from the average of three measurements during a control period of 30 min prior to administering 5.6 mg/kg i.p. apomorphine to both young adult and old gerbils. Each point represents the average change ± SE of five animals. No significant difference was found by the group comparison t test.
Tannen ¡Domino
172
rectal temperatures were monitored during and for some time after the environ mental challenge (fig. 4a, b). No significant difference between the two groups could be detected at any time. However, a peculiar temperature fluctuation could be observed among the old animals placed in a cold room. Apomorphine Hypothermia Dopamine may be involved in the central nervous system regulation of temperature (9, 13, 15). It has been suggested that (+)amphetamine may pro duce its hyperthermic effects via a dopaminergic mechanism (8). To test the possibility that a dopaminergic mechanism might be involved in the exaggerated hyperthermic response to (+)amphetamine in old gerbils, both young adult and old animals were given 5.6 mg/kg of apomorphine, a dopaminergic agonist, and their rectal temperature monitored. No significant difference between the two age groups could be found, although a marked hypothermia was produced along with behavioral excitation (fig. 5).
The experiments presented here demonstrate that old gerbils are more sensi tive than young adults to (+)amphetamine. Old animals show greater locomotor activity, greater hyperthermia and greater toxicity than young adults receiving comparable doses of the drug. It has been further shown that this age-related difference is not due to the amount of the drug which ultimately gets into the blood or brain or to the length of time it remains at these sites. No age-related difference could be found in the ability to regulate body temperature in the face of extreme environmental temperatures nor could a difference be detected in the hypothermic response to the dopaminergic agonist, apomorphine. These observa tions suggest that an inability to regulate temperature is not the cause of amphetamine hypersensitivity in the old gerbil and neither is increased sensi tivity of dopaminergic receptors the probable cause. Since an age-related hyper sensitivity was observed, it is possible that the direct actions of (+)amphetamine are different in old gerbils than in young adults. The findings of this investigation conflict with evidence which has been obtained supporting a resistance to (+)amphetamine stimulation (1 ,4 , 12, 17, 19). This apparent contradiction may ultimately be explained by species differ ences, since such variability is suggested with regard to the peripheral (7) or central (8) mediation of (+)amphetamine hyperthermia. The constellation of exaggerated responses to (+)amphetamine in old gerbils may prove to be interrelated in that higher ambient temperatures are associated with greater locomotor activity (5) and greater toxicity (14) in response to (+)amphetamine. To study this possibility, it would be necessary to dissociate the various components of the pharmacology of (+)amphetamine.
Downloaded by: Univ. of California Santa Barbara 128.111.121.42 - 3/6/2018 5:01:08 PM
Discussion
(+)Amphetamine in Geriatric Gcrbils
173
References
Dr. E.F. Domino, Department of Pharmacology, University of Michigan, Ann Arbor, MI 48109 (USA)
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