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Early housing experience modifies morphine self-administration and physical dependence in adult rats
Addictive Behaviors, Vol. 9. pp. 235-243, 1984 Printed in the USA. All rights reserved.
0306~4603/84 $3.00 + .OO Copyright e 1984 Pergamon Press Ltd
EARLY HOUSING EXPERIENCE MODIFIES MORPHINE SELF-ADMINISTRATION AND PHYSICAL DEPENDENCE IN ADULT RATS ROBIN MARKS-KAUFMAN Tufts University and
MICHAEL
J. LEWIS
Howard University Abstract-Male
Sprague-Dawley rats were raised from weaning in one of three housing conditions: one, two or four per housing unit. At 60 days of age, animals were moved to individual cages and tested in the open field. Following a baseline period in which animals were allowed to adapt to their new housing condition, animals had their water replaced with a 0.8 mg/ml morphine sulfate solution. Following 12 days of access to the drug, animals were injected with naloxone and abstinence precipitated. While no differences were found in body weight among the three groups of animals at 60 days of age, significant differences in open field behavior were noted. Animals that were raised in groups were found to be more active in the open field than animals raised in isolation. Early housing experience was also found to modify later morphine consumption and physical dependence. Animals raised in isolation exhibited a trend to start drinking morphine sooner and experienced less severe withdrawal symptoms following naloxone administration than group-raised animals.
Rats raised in isolation are reported to consume more of a morphine solution than group-housed animals (Alexander, Coambs, & Hadaway, 1978; Hadaway, Alexander, Coambs, & Beyerstein, 1979). It has been suggested that this effect may be due in part to the unnatural and possibly aversive nature of isolated housing (Alexander et al., 1979). Isolated animals may consume morphine, which has both analgesic and anxietyrelieving properties, to ameliorate the stress induced by their housing situation. An alternative hypothesis is that isolated rats consume more morphine than group-housed animals because they are less responsive to the effects of the drug. Isolated animals might therefore have to consume more of a morphine solution to experience the same effects as animals raised in groups. In support of this hypothesis, Katz and Steinberg (1970) found that isolated rats were less sensitive to the depressant actions of injected morphine than were group-housed animals. In addition, rats raised in isolation have been found to be less responsive to morphine as measured by the degree of withdrawal experienced following naloxone-precipitated abstinence (Adler, Bendotti, Ghezzi, Samanin, Valzelli, 1975a). Adler et al. found isolated rats experienced a less severe naloxone-precipitated withdrawal than their group-housed counterparts. In the present study, the effects of raising animals under varying housing conditions on later morphine self-administration and subsequent physical dependence were examined. To eliminate the possibility that the key variable in the animal’s propensity to
This study was partially supported by grants from the Tufts University Faculty Research Award Committee. DHEW (RR07179 and DA 02176) and Bernard Harleston, Dean of Faculty, Tufts University. Reprint re&zsts should be sent to Robin Marks-Kaufman, Institute of Human Nutrition, Columbia University, 630 W. 168th Street, New York, NY 10032. 235
236
ROBIN MARKS-KAUFMAN
and MICHAEL J. LEWIS
drink was the presence or absence of other animals at the time of testing, all animals were housed individually beginning 2 weeks prior to being given access to the drug. METHODS
Animals and apparatus Forty-six 21-day-old male Sprague-Dawley rats (CD outbred, Charles River Breeding Laboratories, Wilmington, MA) were used. Animals were maintained in a temperature-controlled room with a 12-12 hour light-dark cycle (lights on 1900 to 0700 hr.). From 21 to 60 days of age, animals were housed in a wire-mesh enclosure measuring 217 cm x 118 cm x 36 cm. The enclosure was subdivided into 20 units, 19 cm x 23 cm x 36 cm and ten units, 19 cm x 66 cm x 36 cm. The 20 smaller units housed individual animals. Seven of the larger units housed pairs of animals, while the remaining three units housed animals in groups of fours. Each compartment was separated from neighboring compartments by a double wire-mesh wall 3 cm wide, which prevented contact between rats in adjoining units (Figure 1).
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two per cage-
A B
four per cage-
C
one per cage
r;llnm
m2cF[
Fig. 1. Animals were housed either one (A), two (B) or four (C) per unit in the wire-mesh enclosure. Numbers refer to the individual animals. Double walls separated adjoining units.
Morphine self-administration
The open field apparatus squares (Hall, 1934).
231
was a box 90 cm on each side, marked off into 15 cm
Drugs A 0.8 mg/ml morphine sulfate (Merck) solution was used. Morphine was dissolved , in tap water and presented to animals in inverted 250 ml glass bottles with rubber stoppers and non-leaking metal drinking spouts. Animals were given 1.6 mg/kg naloxone hydrochloride (Endo Laboratories, Garden City, NY) to precipitate abstinence. Naloxone was dissolved in distilled water. No animal received more than 1 cc volume of this solution. Procedure Animals were placed in the wire-mesh enclosure at 21 days of age and given ad lib access to food (Purina Rodent Chow No. 5001) and water. At 60 days of age, all animals were weighed and then placed in individual stainless steel cages. Handling while weighing animals was by the tail to minimize physical contact. Animals were given 6 days to acclimate to this new housing condition. Following this adaptation period, each animal was tested once on each of 2 consecutive days in the open field apparatus. During the test period, animals were placed in the open field and the number of lines crossed during a 3-minute interval was recorded. Following the open field test, baseline water and body weight measurements were recorded for 7 days at 1900 hr. Initial self-administration. Following baseline measurements, all animals had their water replaced with a 0.8 mg/ml morphine sulfate solution. Drug intake and body weight data were collected for twelve consecutive days at 1900 hr. Precipitated abstinence. On the 13th day animals were given a 1.6 mg/kg intraperitoneal injection of naloxone to precipitate abstinence. Animals were observed for a 20-minute period and the presence of the following symptoms (Lewis, Costa, Jacobowitz, & Margules, 1976; Wei, Loh, & Way, 1973) recorded: diarrhea, abnormal posturing, ptosis, teeth chattering, swallowing movements, number of escape attempts, number of wet dog shakes, vocalizations when touched, and dyspnea. The morphine solntion was then removed and replaced with water. Relapse. Six weeks following precipitated abstinence, eight of the animals from each housing condition matched for morphine consumption over the 1Zday drug period, had their water again replaced with a 0.8 mg/ml solution of morphine. Body weights and drug intake were recorded daily for three days at 1900 hr. Statistical analyses. All data were analyzed using one-way analyses of variance followed by a posteriori multiple comparison tests, except when the analyses were performed on the proportion of animals exhibiting each of the withdrawal symptoms in which case a Chi-square test was used. RESULTS
No significant differences in body weight were found among the animals in the three housing conditions when they were removed from the wire-mesh enclosure at 60 days of age. However, a significant (p c 0.05) difference in open field activity was found at this time. On both test days, isolated animals were found to be significantly (p < 0.05) less active in the open field than group-housed animals. Animals raised four to a unit were found to be the most active, with pair-housed animals falling in the middle
238
ROBIN I$ARKS-KAUFMAN
and MICHAEL J. LEWIS
HOUSING CONDITION Fig. 2. Mean number of lines crossed in the open field (*S.E.) animals raised either one, two or four per unit.
in each of two test sessions for
(Figure 2). In the second test session, animals were significantly less active in all three housing conditions than during the first test session. Initial self-administration. Over the 12-day morphine period, animals in each of the three housing conditions consumed significantly (p c 0.05) less fluid than during the seven baseline water days (Figure 3). Whereas baseline water intake was fairly stable on a day to day basis, a cyclical pattern of morphine intake was observed in a large percentage of the animals. Individual animals would consume large quantities of the drug solution for 2-3 day periods, and then consume little fluid for 2-3 days. For example, one rat consumed 100 ml of fluid on the first day of its cycle while consuming only 5 ml on the third day. Approximately 80% of the animals exhibited this behavior (representative animals - Figure 4). While over the 12-day period no significant differences were found in morphine intake among animals in the three housing conditions, a trend (p < 0.10) was evident over the first 3 days of access to the drug. Isolates started to drink the morphine solution sooner than group-reared animals (Figure 3). No trends existed during the remainder of this period of self-administration. Precipitated abstinence. Following naloxone injections, a significantly (p < 0.05) larger proportion of the group-housed animals (92% of animals raised in groups of four, 72% of the pair-raised animals) exhibited diarrhea than isolated animals (68%). Additionally, group-housed animals exhibited significantly (p c 0.05) more wet dog
239
Morphine self-administration
WATER
MORPHINE
60 ..
10 t 1
.
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i
4
6
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:
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50
:
0
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W A T E R
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Fig. 3. Mean fluid intake for rats raised one, two or four per unit. Data points represent mean baseline water period, twelve days of access to a 0.8 mg/ml morphine solution as the sole source of fluid and relapse, six weeks following precipitated abstinence when morphine was again the sole source of fluid.
shakes than animals raised in isolation (Figure 5), with animals raised in groups of four experiencing significantly more wet dog shakes than isolates, and pair-housed animals falling in the middle. No other differences were found among the three housing conditions in the number of withdrawal symptoms observed.
Relapse. Six weeks later, when the animals were again given access to the morphine solution, animals in all three groups drank significantly (p c 0.05) more morphine solution on the first day of relapse than on their first day of initial exposure to the drug. Isolated animals consumed significantly (p c 0.05) more drug solution during the relapse period than group-housed animals (Figure 3). Pair-raised animals consumed the least amount of morphine during relapse, with animals raised in groups of four falling in the middle.
240
ROBIN MARKS-KAUFMAN
WATER
and MICHAEL J. LEWIS
MORPHINE
WATER
MORPHINE
l6a
6 6a
llb
m 4 88
DAYS
24681012
DAYS
Fig. 4. Patterns of drinking for six individual animals given water and then a 0.8 mg/ml morphine sulfate solution.
DISCUSSION
Early housing experience modifies later morphine self-administration and physical dependence in rats. Animals raised in isolation exhibited a trend to start drinking morphine sooner and experienced less severe withdrawal symptoms following naloxone administration and consumed more morphine during relapse than group-raised animals. The present study is consistent with data from previous studies in which individuallyhoused animals consumed more of a morphine solution than group-housed animals (Alexander et al., 1978; Hadaway et al., 1979). Moreover, the present experiment makes it appear unlikely that it is either the aversive nature of isolated housing that leads rats to consume more of a morphine solution, or conversely, that it is some aspect of group housing that inhibits morphine intake. In the present experiment, following the initial rearing period, all animals were adapted to and then tested in individual cages. The housing condition employed in the present experiment did not merely compare isolation with group housing, but also compared the effects of varying the degree of physical contact the animal received during development. All rats could hear, see and smell other animals, but physical contact was limited. This is similar to studies in which
Morphine self-administration
241
60
50 P
II
40
8
3 8
30
Y f
20
ul
ONE
TWO HOUSING CONDITION
Fig. 5. Mean number of wet dog shakes ( f S.E.) exhibited by animals raised either one, two or four per unit following the administration of 1.6 mg/kg naloxone hydrochloride.
Harlow and Suomi (1970) physically isolated young monkeys while still having their peers visible. These investigators concluded that bodily contact during development was one of the key variables in helping animals develop a normal adult repertoire. Unlike most deprivation studies which find isolated rats to weigh more (e.g., Krech, Rosenzweig, & Bennet, 1960), no significant differences in body weight were found in the present experiment. While early isolation did not modify body weight, it did have a significant effect on later adult behavior as noted by the animals’ performances in the open field apparatus. Isolates were found to be less active in the open field than pairraised animals, who were, in turn, less active than animals raised in groups of four. This is in agreement with studies which find deprived animals to be less active in the open field (Denenberg & Morton, 1962). These findings in conjunction with those of prior studies lend support to the hypothesis that animals raised in different housing conditions have different behavioral responses to morphine. In particular, isolated rats manifested a less severe withdrawal syndrome than did group-housed animals even though the two groups had consumed equivalent amounts of morphine. In further support of this hypothesis, a study by Katz and Steinberg (1970) noted that isolated rats were less sensitive to the depressant actions intraperitoneal injections of morphine than group-housed animals. Similarly, Kostowski, Czlonkowski, Rewerski, and Piechocki (1977) found that isolated rats who were non-muricidal, as opposed to rats who were mouse killers, were less responsive to the analgesic effects of morphine. In contrast, Adler, Mauron,
242
ROBIN MARKS-KAUFMAN
and MICHAEL J. LEWIS
Samanin and Valzelli (1975b) found that isolation in rats did not alter pain thresholds. However, in another study, Adler et al. (1975a) found isolated rats appeared to behave as if they had received less drug than group-housed animals following morphine pellet implantation. They found that isolated rats made dependent on morphine by pellet implantation exhibited less jumping and less diarrhea during withdrawal precipitated by naloxone than group-housed animals. Unfortunately, while Adler et al. (1975a) noted the presence or absence of wet dog shakes in these animals, this symptom was not quantified. Interesting patterns of drug administration were noted in the present experiment. In contrast to baseline water intake which varied only slightly on a daily basis, oral morphine intake varied substantially across days. When given access to morphine, animals appeared to exhibit 3-4-day cycles in fluid intake. Individual animals were observed to lose or gain up to 30% of their body weight over a 3-day period. McMillan, Leander, Wilson, Wai!ace, Fix, Redding, and Turk (1976) noted animals drinking a 1 mg/ml morphine solution showed increases in intake on the third, eighth and eleventh days of access to the drug, due to disruptions of their normal diurnal pattern of fluid intake. Unfortunately, they did not present data from individual animals. One possible explanation for the cycles in drug intake observed in the present experiment comes from Nichols (1963, 1965). He noted that withdrawal symptoms peaked two to four days after the last oral morphine intake. This coincides with the average length of the cycling period. It is possible that animals self-administered large quantities of drug with peak of withdrawal. Abnormally high intake at the beginning of cycles may be in part due to the slower onset of the post-ingestional effects of morphine when orally administered than when administered intravenously. If the drug was not immediately reinforcing, the animal might drink large quantities of the solution until the reinforcing properties took effect. Animals might then not administer the drug until the aversive consequences of withdrawal were realized. It is interesting to note that monkeys intravenously self-administering morphine do not show this irregular pattern of drug intake (Deneau, Yanagita, & Seevers, 1969). The cycling pattern of oral morphine intake, the degree of self-regulation it entails and its relationship to the withdrawal syndrome are all areas which require further investigation. REFERENCES Adler, M.W., Bendotti, G., Ghezzi, D., Samanin, R., & Valzelli, L. Dependence to morphine in differentially housed rats. Psychophurmacologia (Berl.), 1975, 41, 15-18.(a) Adler, M.W., Mauron, C., Samanin, R., & Valzelli, L. Morphine analgesia in grouped and isolated rats. Psychophurmucologia (Berl.), 1975, 41, 11-14.(b) Alexander B.K., Coambs, R.B., & Hadaway, P.F. The effects of housing and gender on morphine selfadministration in rats. Psychopharmacology, 1978, 58, 175-179. Deneau, GA., Yanagita, T., & Seevers, M.H. Self-administration of psychoactive substances by the monkey. Psychopharmacologia. 1%9, 16, 30-48. Denenberg, V.H., & Morton, R.R.C. Effects of environmental complexity and social groupings upon modification of emotional behavior. Journal of Comparative and Physiological Psychology, 1962, 55, 242-246. Hadaway, P.F., Alexander, B.K., Coambs, R.B., & Beyerstein, B. The effects of housing and gender on preference for morphine-sucrose solutions in rats. Psychopharmacology, 1979, 66, 87-91. Hall, C.S. Emotional behavior in the rat. Journal of Comparative Psychology, 1934, 18, 385-403. Harlow, H.F. and Suomi, S.J. Nature of love simplified. American Psychologist, 1970, 25, 161-168. Katz, D.M., & Steinberg, H. Long-term isolation in rats reduces morphine response. Nature (Lon.), 1972, 228.469-47 1. Kostowski, W., Czlonkowski, A., Rewerski, W., SLPiechocki, T. Morphine action in grouped and isolated rats and mice. Psychopharmacologia, 1977, 53, 191-193.
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243
Krech, D., Rosenzweig, M.R., & Bennet, E.F. Effects of environment complexity and training on brain chemistry. Journal of Comparative and Physiological Psychology, 1960, 53, 509-519. Lewis, M.J., Costa, J.L., Jacobowitz, D., & Margules, D.L. Tolerance, physical dependence and opioidseeking behavior: Dependence on diencephalic norepinepherine. Brain Research, 1976, 107, 156-167. McMillan, D.E., Leander, J.D., Wilson, T. W., Wallace, SC., Fox, T., Redding, S., & Turk, R.T. Oral ingestion of narcotic analgesics by rats. Journal of Pharmacology and Experimental Therapeutics, 1976, 196, 269-279. Nichols, J.R. A procedure which produces sustained opiate-directed behavior in the rat. Psychological Reports, 1963, 13, 895-904. Nichols, J.R. How opiates change behavior. Scientific American, 1%5, 212, 80-88. Wei, E., Loh H.H., & Way, E.L. Quantitative aspects of precipitated abstinence in morphine-dependent rats. Journal of Pharmacology and Experimental Therapeutics, 1973, 184, 398-403.
0306~4603/84 $3.00 + .OO Copyright e 1984 Pergamon Press Ltd
EARLY HOUSING EXPERIENCE MODIFIES MORPHINE SELF-ADMINISTRATION AND PHYSICAL DEPENDENCE IN ADULT RATS ROBIN MARKS-KAUFMAN Tufts University and
MICHAEL
J. LEWIS
Howard University Abstract-Male
Sprague-Dawley rats were raised from weaning in one of three housing conditions: one, two or four per housing unit. At 60 days of age, animals were moved to individual cages and tested in the open field. Following a baseline period in which animals were allowed to adapt to their new housing condition, animals had their water replaced with a 0.8 mg/ml morphine sulfate solution. Following 12 days of access to the drug, animals were injected with naloxone and abstinence precipitated. While no differences were found in body weight among the three groups of animals at 60 days of age, significant differences in open field behavior were noted. Animals that were raised in groups were found to be more active in the open field than animals raised in isolation. Early housing experience was also found to modify later morphine consumption and physical dependence. Animals raised in isolation exhibited a trend to start drinking morphine sooner and experienced less severe withdrawal symptoms following naloxone administration than group-raised animals.
Rats raised in isolation are reported to consume more of a morphine solution than group-housed animals (Alexander, Coambs, & Hadaway, 1978; Hadaway, Alexander, Coambs, & Beyerstein, 1979). It has been suggested that this effect may be due in part to the unnatural and possibly aversive nature of isolated housing (Alexander et al., 1979). Isolated animals may consume morphine, which has both analgesic and anxietyrelieving properties, to ameliorate the stress induced by their housing situation. An alternative hypothesis is that isolated rats consume more morphine than group-housed animals because they are less responsive to the effects of the drug. Isolated animals might therefore have to consume more of a morphine solution to experience the same effects as animals raised in groups. In support of this hypothesis, Katz and Steinberg (1970) found that isolated rats were less sensitive to the depressant actions of injected morphine than were group-housed animals. In addition, rats raised in isolation have been found to be less responsive to morphine as measured by the degree of withdrawal experienced following naloxone-precipitated abstinence (Adler, Bendotti, Ghezzi, Samanin, Valzelli, 1975a). Adler et al. found isolated rats experienced a less severe naloxone-precipitated withdrawal than their group-housed counterparts. In the present study, the effects of raising animals under varying housing conditions on later morphine self-administration and subsequent physical dependence were examined. To eliminate the possibility that the key variable in the animal’s propensity to
This study was partially supported by grants from the Tufts University Faculty Research Award Committee. DHEW (RR07179 and DA 02176) and Bernard Harleston, Dean of Faculty, Tufts University. Reprint re&zsts should be sent to Robin Marks-Kaufman, Institute of Human Nutrition, Columbia University, 630 W. 168th Street, New York, NY 10032. 235
236
ROBIN MARKS-KAUFMAN
and MICHAEL J. LEWIS
drink was the presence or absence of other animals at the time of testing, all animals were housed individually beginning 2 weeks prior to being given access to the drug. METHODS
Animals and apparatus Forty-six 21-day-old male Sprague-Dawley rats (CD outbred, Charles River Breeding Laboratories, Wilmington, MA) were used. Animals were maintained in a temperature-controlled room with a 12-12 hour light-dark cycle (lights on 1900 to 0700 hr.). From 21 to 60 days of age, animals were housed in a wire-mesh enclosure measuring 217 cm x 118 cm x 36 cm. The enclosure was subdivided into 20 units, 19 cm x 23 cm x 36 cm and ten units, 19 cm x 66 cm x 36 cm. The 20 smaller units housed individual animals. Seven of the larger units housed pairs of animals, while the remaining three units housed animals in groups of fours. Each compartment was separated from neighboring compartments by a double wire-mesh wall 3 cm wide, which prevented contact between rats in adjoining units (Figure 1).
q 15A
Il6A II
116
128
I II 19A
I”
lOB
9B
1(5C
6C
7C
q
6CI 1
12A
HA]
HOUSING
two per cage-
A B
four per cage-
C
one per cage
r;llnm
m2cF[
Fig. 1. Animals were housed either one (A), two (B) or four (C) per unit in the wire-mesh enclosure. Numbers refer to the individual animals. Double walls separated adjoining units.
Morphine self-administration
The open field apparatus squares (Hall, 1934).
231
was a box 90 cm on each side, marked off into 15 cm
Drugs A 0.8 mg/ml morphine sulfate (Merck) solution was used. Morphine was dissolved , in tap water and presented to animals in inverted 250 ml glass bottles with rubber stoppers and non-leaking metal drinking spouts. Animals were given 1.6 mg/kg naloxone hydrochloride (Endo Laboratories, Garden City, NY) to precipitate abstinence. Naloxone was dissolved in distilled water. No animal received more than 1 cc volume of this solution. Procedure Animals were placed in the wire-mesh enclosure at 21 days of age and given ad lib access to food (Purina Rodent Chow No. 5001) and water. At 60 days of age, all animals were weighed and then placed in individual stainless steel cages. Handling while weighing animals was by the tail to minimize physical contact. Animals were given 6 days to acclimate to this new housing condition. Following this adaptation period, each animal was tested once on each of 2 consecutive days in the open field apparatus. During the test period, animals were placed in the open field and the number of lines crossed during a 3-minute interval was recorded. Following the open field test, baseline water and body weight measurements were recorded for 7 days at 1900 hr. Initial self-administration. Following baseline measurements, all animals had their water replaced with a 0.8 mg/ml morphine sulfate solution. Drug intake and body weight data were collected for twelve consecutive days at 1900 hr. Precipitated abstinence. On the 13th day animals were given a 1.6 mg/kg intraperitoneal injection of naloxone to precipitate abstinence. Animals were observed for a 20-minute period and the presence of the following symptoms (Lewis, Costa, Jacobowitz, & Margules, 1976; Wei, Loh, & Way, 1973) recorded: diarrhea, abnormal posturing, ptosis, teeth chattering, swallowing movements, number of escape attempts, number of wet dog shakes, vocalizations when touched, and dyspnea. The morphine solntion was then removed and replaced with water. Relapse. Six weeks following precipitated abstinence, eight of the animals from each housing condition matched for morphine consumption over the 1Zday drug period, had their water again replaced with a 0.8 mg/ml solution of morphine. Body weights and drug intake were recorded daily for three days at 1900 hr. Statistical analyses. All data were analyzed using one-way analyses of variance followed by a posteriori multiple comparison tests, except when the analyses were performed on the proportion of animals exhibiting each of the withdrawal symptoms in which case a Chi-square test was used. RESULTS
No significant differences in body weight were found among the animals in the three housing conditions when they were removed from the wire-mesh enclosure at 60 days of age. However, a significant (p c 0.05) difference in open field activity was found at this time. On both test days, isolated animals were found to be significantly (p < 0.05) less active in the open field than group-housed animals. Animals raised four to a unit were found to be the most active, with pair-housed animals falling in the middle
238
ROBIN I$ARKS-KAUFMAN
and MICHAEL J. LEWIS
HOUSING CONDITION Fig. 2. Mean number of lines crossed in the open field (*S.E.) animals raised either one, two or four per unit.
in each of two test sessions for
(Figure 2). In the second test session, animals were significantly less active in all three housing conditions than during the first test session. Initial self-administration. Over the 12-day morphine period, animals in each of the three housing conditions consumed significantly (p c 0.05) less fluid than during the seven baseline water days (Figure 3). Whereas baseline water intake was fairly stable on a day to day basis, a cyclical pattern of morphine intake was observed in a large percentage of the animals. Individual animals would consume large quantities of the drug solution for 2-3 day periods, and then consume little fluid for 2-3 days. For example, one rat consumed 100 ml of fluid on the first day of its cycle while consuming only 5 ml on the third day. Approximately 80% of the animals exhibited this behavior (representative animals - Figure 4). While over the 12-day period no significant differences were found in morphine intake among animals in the three housing conditions, a trend (p < 0.10) was evident over the first 3 days of access to the drug. Isolates started to drink the morphine solution sooner than group-reared animals (Figure 3). No trends existed during the remainder of this period of self-administration. Precipitated abstinence. Following naloxone injections, a significantly (p < 0.05) larger proportion of the group-housed animals (92% of animals raised in groups of four, 72% of the pair-raised animals) exhibited diarrhea than isolated animals (68%). Additionally, group-housed animals exhibited significantly (p c 0.05) more wet dog
239
Morphine self-administration
WATER
MORPHINE
60 ..
10 t 1
.
:
;
i
4
6
1
:
:
:
!
:
:
2
4
6
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12
DAYS
IELAPSE
50
:
0
E i=
40
$z In 5 r 0
30 20
W A T E R
10 -I-
+ MEAN
2 DAYS
Fig. 3. Mean fluid intake for rats raised one, two or four per unit. Data points represent mean baseline water period, twelve days of access to a 0.8 mg/ml morphine solution as the sole source of fluid and relapse, six weeks following precipitated abstinence when morphine was again the sole source of fluid.
shakes than animals raised in isolation (Figure 5), with animals raised in groups of four experiencing significantly more wet dog shakes than isolates, and pair-housed animals falling in the middle. No other differences were found among the three housing conditions in the number of withdrawal symptoms observed.
Relapse. Six weeks later, when the animals were again given access to the morphine solution, animals in all three groups drank significantly (p c 0.05) more morphine solution on the first day of relapse than on their first day of initial exposure to the drug. Isolated animals consumed significantly (p c 0.05) more drug solution during the relapse period than group-housed animals (Figure 3). Pair-raised animals consumed the least amount of morphine during relapse, with animals raised in groups of four falling in the middle.
240
ROBIN MARKS-KAUFMAN
WATER
and MICHAEL J. LEWIS
MORPHINE
WATER
MORPHINE
l6a
6 6a
llb
m 4 88
DAYS
24681012
DAYS
Fig. 4. Patterns of drinking for six individual animals given water and then a 0.8 mg/ml morphine sulfate solution.
DISCUSSION
Early housing experience modifies later morphine self-administration and physical dependence in rats. Animals raised in isolation exhibited a trend to start drinking morphine sooner and experienced less severe withdrawal symptoms following naloxone administration and consumed more morphine during relapse than group-raised animals. The present study is consistent with data from previous studies in which individuallyhoused animals consumed more of a morphine solution than group-housed animals (Alexander et al., 1978; Hadaway et al., 1979). Moreover, the present experiment makes it appear unlikely that it is either the aversive nature of isolated housing that leads rats to consume more of a morphine solution, or conversely, that it is some aspect of group housing that inhibits morphine intake. In the present experiment, following the initial rearing period, all animals were adapted to and then tested in individual cages. The housing condition employed in the present experiment did not merely compare isolation with group housing, but also compared the effects of varying the degree of physical contact the animal received during development. All rats could hear, see and smell other animals, but physical contact was limited. This is similar to studies in which
Morphine self-administration
241
60
50 P
II
40
8
3 8
30
Y f
20
ul
ONE
TWO HOUSING CONDITION
Fig. 5. Mean number of wet dog shakes ( f S.E.) exhibited by animals raised either one, two or four per unit following the administration of 1.6 mg/kg naloxone hydrochloride.
Harlow and Suomi (1970) physically isolated young monkeys while still having their peers visible. These investigators concluded that bodily contact during development was one of the key variables in helping animals develop a normal adult repertoire. Unlike most deprivation studies which find isolated rats to weigh more (e.g., Krech, Rosenzweig, & Bennet, 1960), no significant differences in body weight were found in the present experiment. While early isolation did not modify body weight, it did have a significant effect on later adult behavior as noted by the animals’ performances in the open field apparatus. Isolates were found to be less active in the open field than pairraised animals, who were, in turn, less active than animals raised in groups of four. This is in agreement with studies which find deprived animals to be less active in the open field (Denenberg & Morton, 1962). These findings in conjunction with those of prior studies lend support to the hypothesis that animals raised in different housing conditions have different behavioral responses to morphine. In particular, isolated rats manifested a less severe withdrawal syndrome than did group-housed animals even though the two groups had consumed equivalent amounts of morphine. In further support of this hypothesis, a study by Katz and Steinberg (1970) noted that isolated rats were less sensitive to the depressant actions intraperitoneal injections of morphine than group-housed animals. Similarly, Kostowski, Czlonkowski, Rewerski, and Piechocki (1977) found that isolated rats who were non-muricidal, as opposed to rats who were mouse killers, were less responsive to the analgesic effects of morphine. In contrast, Adler, Mauron,
242
ROBIN MARKS-KAUFMAN
and MICHAEL J. LEWIS
Samanin and Valzelli (1975b) found that isolation in rats did not alter pain thresholds. However, in another study, Adler et al. (1975a) found isolated rats appeared to behave as if they had received less drug than group-housed animals following morphine pellet implantation. They found that isolated rats made dependent on morphine by pellet implantation exhibited less jumping and less diarrhea during withdrawal precipitated by naloxone than group-housed animals. Unfortunately, while Adler et al. (1975a) noted the presence or absence of wet dog shakes in these animals, this symptom was not quantified. Interesting patterns of drug administration were noted in the present experiment. In contrast to baseline water intake which varied only slightly on a daily basis, oral morphine intake varied substantially across days. When given access to morphine, animals appeared to exhibit 3-4-day cycles in fluid intake. Individual animals were observed to lose or gain up to 30% of their body weight over a 3-day period. McMillan, Leander, Wilson, Wai!ace, Fix, Redding, and Turk (1976) noted animals drinking a 1 mg/ml morphine solution showed increases in intake on the third, eighth and eleventh days of access to the drug, due to disruptions of their normal diurnal pattern of fluid intake. Unfortunately, they did not present data from individual animals. One possible explanation for the cycles in drug intake observed in the present experiment comes from Nichols (1963, 1965). He noted that withdrawal symptoms peaked two to four days after the last oral morphine intake. This coincides with the average length of the cycling period. It is possible that animals self-administered large quantities of drug with peak of withdrawal. Abnormally high intake at the beginning of cycles may be in part due to the slower onset of the post-ingestional effects of morphine when orally administered than when administered intravenously. If the drug was not immediately reinforcing, the animal might drink large quantities of the solution until the reinforcing properties took effect. Animals might then not administer the drug until the aversive consequences of withdrawal were realized. It is interesting to note that monkeys intravenously self-administering morphine do not show this irregular pattern of drug intake (Deneau, Yanagita, & Seevers, 1969). The cycling pattern of oral morphine intake, the degree of self-regulation it entails and its relationship to the withdrawal syndrome are all areas which require further investigation. REFERENCES Adler, M.W., Bendotti, G., Ghezzi, D., Samanin, R., & Valzelli, L. Dependence to morphine in differentially housed rats. Psychophurmacologia (Berl.), 1975, 41, 15-18.(a) Adler, M.W., Mauron, C., Samanin, R., & Valzelli, L. Morphine analgesia in grouped and isolated rats. Psychophurmucologia (Berl.), 1975, 41, 11-14.(b) Alexander B.K., Coambs, R.B., & Hadaway, P.F. The effects of housing and gender on morphine selfadministration in rats. Psychopharmacology, 1978, 58, 175-179. Deneau, GA., Yanagita, T., & Seevers, M.H. Self-administration of psychoactive substances by the monkey. Psychopharmacologia. 1%9, 16, 30-48. Denenberg, V.H., & Morton, R.R.C. Effects of environmental complexity and social groupings upon modification of emotional behavior. Journal of Comparative and Physiological Psychology, 1962, 55, 242-246. Hadaway, P.F., Alexander, B.K., Coambs, R.B., & Beyerstein, B. The effects of housing and gender on preference for morphine-sucrose solutions in rats. Psychopharmacology, 1979, 66, 87-91. Hall, C.S. Emotional behavior in the rat. Journal of Comparative Psychology, 1934, 18, 385-403. Harlow, H.F. and Suomi, S.J. Nature of love simplified. American Psychologist, 1970, 25, 161-168. Katz, D.M., & Steinberg, H. Long-term isolation in rats reduces morphine response. Nature (Lon.), 1972, 228.469-47 1. Kostowski, W., Czlonkowski, A., Rewerski, W., SLPiechocki, T. Morphine action in grouped and isolated rats and mice. Psychopharmacologia, 1977, 53, 191-193.
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Krech, D., Rosenzweig, M.R., & Bennet, E.F. Effects of environment complexity and training on brain chemistry. Journal of Comparative and Physiological Psychology, 1960, 53, 509-519. Lewis, M.J., Costa, J.L., Jacobowitz, D., & Margules, D.L. Tolerance, physical dependence and opioidseeking behavior: Dependence on diencephalic norepinepherine. Brain Research, 1976, 107, 156-167. McMillan, D.E., Leander, J.D., Wilson, T. W., Wallace, SC., Fox, T., Redding, S., & Turk, R.T. Oral ingestion of narcotic analgesics by rats. Journal of Pharmacology and Experimental Therapeutics, 1976, 196, 269-279. Nichols, J.R. A procedure which produces sustained opiate-directed behavior in the rat. Psychological Reports, 1963, 13, 895-904. Nichols, J.R. How opiates change behavior. Scientific American, 1%5, 212, 80-88. Wei, E., Loh H.H., & Way, E.L. Quantitative aspects of precipitated abstinence in morphine-dependent rats. Journal of Pharmacology and Experimental Therapeutics, 1973, 184, 398-403.