Psychopharmacology(1992) 107:277-284
Psychopharmacology © Springer-Verlag 1992
Locomotor-activating effects of the D2 agonist bromocriptine show environment-specific sensitization following repeated injections Diane C. Hoffman and Roy A. Wise Center for Studies in Behavioral Neurobiology,Department of Psychology,Concordia University, 1455 de MaisonneuveBoulevardW., Montreal, Canada H3G 1M8 ReceivedApril 15, 1991 / Final version August 20, 1991 Abstract. Biphasic effects of bromocriptine (2.0, 5.0, 10.0, and 20.0 mg/kg IP) on locomotion were quantified in photocell activity boxes in rats. Following early suppression of activity, bromocriptine produced a clear, dose-dependent increase in locomotion that lasted several hours. When a low dose of bromocriptine (5.0 mg/kg) was administered daily over a 3-week period, the locomotor-activating effects of the drug showed progressive enhancement over days. The sensitization was environment specific; rats administered bromocriptine six times in the home cage showed no sign of a sensitized response to bromocriptine when subsequently tested in the activity box. Thus, selective stimulation of Dz receptors stimulates locomotion and sensitizes animals to subsequent injections, just as do the indirect-acting dopamine agonists cocaine and amphetamine. Key words: Bromocriptine- Conditioning- D 2 receptors - Dopamine - Locomotion - Sensitization
Psychomotor stimulants and opiates increase locomotor activity, and progressively larger increases in activity are observed with successive injections; this progressive increase is referred to as "behavioral sensitization." A variety of drugs that cause dopamine (DA) release or block DA reuptake (Di Chiara and Imperato 1988) produce such sensitization; these include amphetamine (Robinson 1984; Drew and Glick 1989; for reviews, see Robinson and Becker 1986 and Stewart and Vezina 1988), methamphetamine (Tadokoro and Kuribara 1986), cocaine (for review, see Post and Contel 1983), morphine (Bartoletti et al. 1983; Kalivas and Duffy 1987) and intra-ventral tegrnental area (VTA) injections of opiates and opioid peptides, such as D-AlaZ-Met5-enkephalinamide (DALA) and Tyr-D-Ala-Gly-NMe-PheGly-ol (DAGO) (Joyce and Iversen 1979; Vezina and OffprOu requests to: D.C. Hoffman, Neurogen Corporation, 35 Northeast Industrial Road, Branford, CT 06405, USA
Stewart 1984; Kalivas 1985; Kalivas et al. 1985; Vezina et al. t987). DA circuitry is involved in locomotor activation and it is widely thought that modifications of the mesostriatal DA system underly the development of psychomotor stimulant- and opiate-induced behavioral sensitization (for reviews see Robinson and Becker 1986; Stewart and Vezina 1988). Neurochemical studies, using both in vitro and in vivo preparations, demonstrate that amphetamine-sensitized rats show enhanced striatal DA release in response to an amphetamine challenge (Robinson and Becker 1982; Robinson et al. 1988). Similarly, rats sensitized to systemic or intra-VTA opiates show enhanced release, synthesis, and metabolism of DA in the nucleus accumbens following an acute injection of the sensitizing drug (Kalivas 1985; Kalivas and Duffy 1987). These drug-induced increases in DA synthesis and release at nerve terminals accompany decreases in synthesis and release in the cell body region (Kalivas and Duffy 1987). Finally, it is the response to the pharmacological stimulus that is enhanced in the sensitized animal; no changes in steady~state levels of DA or DA metabolites are found (Kalivas 1985; Robinson et al. 1988). It is also known that some form of DA receptor activation is necessary for sensitization to occur because blockade of DA receptors can prevent the establishment of stimulant- or opiate-induced behavioral sensitization. For example, pretreatment with the preferential D2 receptor antagonist haloperidol or the selective D1 receptor antagonist SCH 23390 blocks the development of amphetamine- and methamphetamine-induced behavioral sensitization (Kuczenski and Leith 1981; Ujike et al. 1989; Vezina and Stewart 1989; Drew and Glick t990). Microinjections of SCH 23390 into either the VTA or substantia nigra pars reticulata prevent the establishment of amphetamine-induced sensitization (Stewart and Vezina 1989). Pretreatment with the D2 receptor antagonist YM-09151-2 blocked the development of sensitization to methamphetamine according to some investigators (Ujike et al. 1989); however, other investigators have failed to block amphetamine-induced sensitization
278 with Dz receptor antagonists (Vezina and Stewart 1989; Drew and Glick 1990). Sensitization produced by VTA morphine injections is blocked by pretreatment with the D2 receptor antagonist pimozide (Vezina and Stewart 1984, 1989, but see Kativas 1985); selective D1 or other Dz receptor antagonists have not been effective in preventing morphine sensitization, but in each case only one dose of the antagonist has been tested (Vezina and Stewart 1989). The importance of DA receptor activation is underscored by the finding that the direct acting DA receptor agonist apomorphine produces an amphetamine-like locomotor sensitization (Mattingly et al. 1988). Sensitization is not a simple consequence of pharmacological activation of DA receptors, however; two findings indicate that an important component of sensitization involves the conditioning of a Pavlovian association between the test drug and the test environment. First, following repeated pairings of a psychomotor stimulant or opiate with a particular environment, the stimuli of the test box can elicit conditioned increases in activity even when the drug is omitted (Post et al. 1981 ; Beninger and H a h n 1983; Vezina and Stewart 1984; Beninger and Herz 1986; Weiss et al. 1989). Second, evidence for behavioral sensitization is strongest when the animal is tested in the same environment in which the drug was repeatedly administered (Post et al. 1981; Vezina and Stewart I984; Drew and Glick 1989). The study of stimulant sensitization has focused largely on drugs that increase synaptic DA concentrations (DiChiara and Imperato 1988; Robinson et al. 1988). Amphetamine is a DA releaser and cocaine a DA uptake inhibitor (Heikkila et al. 1975); morphine increases DA release by increasing DA impulse flow (Ostrowski et al. 1982; Matthews and German 1984). Because increased synaptic concentrations of DA result in nonselective stimulation of DA receptor subtypes (D1 and D2), the relative importance of these receptor subtypes in the development of sensitization is unknown. There are a few studies that have examined the effects of selective DA receptor antagonists o n the development of amphetamine- or morphine-induced sensitization but the results are not entirely consistent (see above); it appears likely that each receptor subtype is important. There is little evidence suggesting sensitization with subtype-selective agonists. Bromocriptine, a direct acting DA receptor agonist, is of interest in this regard for two reasons. First, this drug shows high affinity for D2 receptors, but low affinity for D1 receptors (Markstein 1981). Second, there is some evidence suggesting that locomotor activation and sensitization may occur with this compound; repeated injections of bromocriptine progressively enhance wheel-running in mice (Smith et al. 1979) and enhance circling in rats with unilateral 6-hydroxydopamine (6-OHDA) lesions of the substantia nigra (Traub et al. 1985; Hassan et al. 1986). However, the ability of bromocriptine to produce locomotor activation and sensitization, particularly in intact rats, has not been well characterized. The present experiments were designed to investigate these phenomena; a systematic analysis of the role of conditioning factors in the development of behav-
ioral sensitization was included. Characterization of bromocriptine-induced sensitization is of clinical as well as theoretical interest, since this drug is an effective therapeutic agent in the treatment of Parkinson's disease.
Experiment 1 In the first experiment the acute locomotor effects of bromocriptine were determined using standard photocell activity boxes. A 10-fold range of doses was explored, and 6-h tests were used to determine the time-course of the effects. Periodic observations of the animals were made to detect signs of stereotypic head or mouth movements that are typical of both indirect and non-selective DA agonists.
Materials and methods Subjects. Twelve male Long-Evans rats (Charles River Canada) weighing 400-500 g were housed individually in a temperaturecontrolled (21:kl °) colony room on a normal 12-h light (0800-2000)/dark cycle and served as subjects. The rats had free access to food and water. Apparatus. Activity was measured in 12 activity boxes (40.5 x 20.5 x 24.5 cm) equipped with two infrared photocell beams and detectors (at a height of 3.3 cm) which divided the length of each box into three equal areas. Each box was constructed of wood with a plexiglas front and rod floor. Ambient sounds were masked by white noise during testing. Drugs. Bromocriptine mesylate (Sandoz, Canada) was dissolved in a solution of 6% mulgofen EL-7t9 (Alkaril Chemicals Ltd, Canada) and 12% ethanol (95%) in physiological saline. The drug was injected IP 30 min prior to testing in a volume of 1 ml/kg. Procedure. The rats were tested in the activity boxes twice weekly, on Mondays and Fridays. They were habituated to the activity boxes for 3 h on the first exposure; on subsequent days, the rats were removed from their home cage and taken to a holding room where they were treated with one of four doses (2.0, 5.0, 10.0 or 20.0 mg/kg IP) of bromocriptine or its vehicle and where they remained for 30 min. The rats were then placed in the activity box located in another room and activity was recorded for 6 h. Each rat was tested with each dose and vehicle; the sequence of dosing was different for each animal.
Results and discussion
Bromocriptine had a biphasic effect on locomotor activity (Fig. 1). Following suppressed activity in the first 30 min, bromocriptine produced a dose-dependent and long-lasting increase in locomotion. A two-way analysis of variance (ANOVA) revealed a significant dose by time interaction [F(44, 484) = 5.85, P < 0.001]. During the first 30 min, each dose of bromocriptine suppressed locomotor activity relative to vehicle treatment (P,. I,-
>200 -
I--
N
300
300 " O 250 -
250 I-.
200 -
O
O 100 -
100 2:
200150 -
15o -
Z
50-
LU
ILl =E
0 30
60
90 TIME
120
150
180
(min)
500
•
,
, 30
•
,
, so
. . . . . . 90 TIME
, • 120
• , 150
,
• , 1so
(rain)
Fig. 4. Mean (± SEM) activity counts at 10-min intervals in the vehicle @[]--), paired (--II--), and unpaired groups (-a--) during the first, third, and sixth conditioning sessions. Only the paired group received bromocriptineduring activity testing
Fig. 5. Mean (+SEM) activity counts at 10-min intervals in the vehicle (--G-), paired (- B--), and unpaired groups (--zx--) during the first, third, and sixth test sessions. All groups were treated with bromocriptine during activity testing
group. The Unpaired group, which received the same number of bromocriptine injections as the Paired group but in a different environment, demonstrated an activity level similar to that observed in the Vehicle group (Fig. 5). A three-way ANOVA conducted on the first, third and sixth test sessions yielded a significant time by group interaction [F(34, 357)=6.4, PI-I j,..
loo.
O
Psychopharmacology © Springer-Verlag 1992
Locomotor-activating effects of the D2 agonist bromocriptine show environment-specific sensitization following repeated injections Diane C. Hoffman and Roy A. Wise Center for Studies in Behavioral Neurobiology,Department of Psychology,Concordia University, 1455 de MaisonneuveBoulevardW., Montreal, Canada H3G 1M8 ReceivedApril 15, 1991 / Final version August 20, 1991 Abstract. Biphasic effects of bromocriptine (2.0, 5.0, 10.0, and 20.0 mg/kg IP) on locomotion were quantified in photocell activity boxes in rats. Following early suppression of activity, bromocriptine produced a clear, dose-dependent increase in locomotion that lasted several hours. When a low dose of bromocriptine (5.0 mg/kg) was administered daily over a 3-week period, the locomotor-activating effects of the drug showed progressive enhancement over days. The sensitization was environment specific; rats administered bromocriptine six times in the home cage showed no sign of a sensitized response to bromocriptine when subsequently tested in the activity box. Thus, selective stimulation of Dz receptors stimulates locomotion and sensitizes animals to subsequent injections, just as do the indirect-acting dopamine agonists cocaine and amphetamine. Key words: Bromocriptine- Conditioning- D 2 receptors - Dopamine - Locomotion - Sensitization
Psychomotor stimulants and opiates increase locomotor activity, and progressively larger increases in activity are observed with successive injections; this progressive increase is referred to as "behavioral sensitization." A variety of drugs that cause dopamine (DA) release or block DA reuptake (Di Chiara and Imperato 1988) produce such sensitization; these include amphetamine (Robinson 1984; Drew and Glick 1989; for reviews, see Robinson and Becker 1986 and Stewart and Vezina 1988), methamphetamine (Tadokoro and Kuribara 1986), cocaine (for review, see Post and Contel 1983), morphine (Bartoletti et al. 1983; Kalivas and Duffy 1987) and intra-ventral tegrnental area (VTA) injections of opiates and opioid peptides, such as D-AlaZ-Met5-enkephalinamide (DALA) and Tyr-D-Ala-Gly-NMe-PheGly-ol (DAGO) (Joyce and Iversen 1979; Vezina and OffprOu requests to: D.C. Hoffman, Neurogen Corporation, 35 Northeast Industrial Road, Branford, CT 06405, USA
Stewart 1984; Kalivas 1985; Kalivas et al. 1985; Vezina et al. t987). DA circuitry is involved in locomotor activation and it is widely thought that modifications of the mesostriatal DA system underly the development of psychomotor stimulant- and opiate-induced behavioral sensitization (for reviews see Robinson and Becker 1986; Stewart and Vezina 1988). Neurochemical studies, using both in vitro and in vivo preparations, demonstrate that amphetamine-sensitized rats show enhanced striatal DA release in response to an amphetamine challenge (Robinson and Becker 1982; Robinson et al. 1988). Similarly, rats sensitized to systemic or intra-VTA opiates show enhanced release, synthesis, and metabolism of DA in the nucleus accumbens following an acute injection of the sensitizing drug (Kalivas 1985; Kalivas and Duffy 1987). These drug-induced increases in DA synthesis and release at nerve terminals accompany decreases in synthesis and release in the cell body region (Kalivas and Duffy 1987). Finally, it is the response to the pharmacological stimulus that is enhanced in the sensitized animal; no changes in steady~state levels of DA or DA metabolites are found (Kalivas 1985; Robinson et al. 1988). It is also known that some form of DA receptor activation is necessary for sensitization to occur because blockade of DA receptors can prevent the establishment of stimulant- or opiate-induced behavioral sensitization. For example, pretreatment with the preferential D2 receptor antagonist haloperidol or the selective D1 receptor antagonist SCH 23390 blocks the development of amphetamine- and methamphetamine-induced behavioral sensitization (Kuczenski and Leith 1981; Ujike et al. 1989; Vezina and Stewart 1989; Drew and Glick t990). Microinjections of SCH 23390 into either the VTA or substantia nigra pars reticulata prevent the establishment of amphetamine-induced sensitization (Stewart and Vezina 1989). Pretreatment with the D2 receptor antagonist YM-09151-2 blocked the development of sensitization to methamphetamine according to some investigators (Ujike et al. 1989); however, other investigators have failed to block amphetamine-induced sensitization
278 with Dz receptor antagonists (Vezina and Stewart 1989; Drew and Glick 1990). Sensitization produced by VTA morphine injections is blocked by pretreatment with the D2 receptor antagonist pimozide (Vezina and Stewart 1984, 1989, but see Kativas 1985); selective D1 or other Dz receptor antagonists have not been effective in preventing morphine sensitization, but in each case only one dose of the antagonist has been tested (Vezina and Stewart 1989). The importance of DA receptor activation is underscored by the finding that the direct acting DA receptor agonist apomorphine produces an amphetamine-like locomotor sensitization (Mattingly et al. 1988). Sensitization is not a simple consequence of pharmacological activation of DA receptors, however; two findings indicate that an important component of sensitization involves the conditioning of a Pavlovian association between the test drug and the test environment. First, following repeated pairings of a psychomotor stimulant or opiate with a particular environment, the stimuli of the test box can elicit conditioned increases in activity even when the drug is omitted (Post et al. 1981 ; Beninger and H a h n 1983; Vezina and Stewart 1984; Beninger and Herz 1986; Weiss et al. 1989). Second, evidence for behavioral sensitization is strongest when the animal is tested in the same environment in which the drug was repeatedly administered (Post et al. 1981; Vezina and Stewart I984; Drew and Glick 1989). The study of stimulant sensitization has focused largely on drugs that increase synaptic DA concentrations (DiChiara and Imperato 1988; Robinson et al. 1988). Amphetamine is a DA releaser and cocaine a DA uptake inhibitor (Heikkila et al. 1975); morphine increases DA release by increasing DA impulse flow (Ostrowski et al. 1982; Matthews and German 1984). Because increased synaptic concentrations of DA result in nonselective stimulation of DA receptor subtypes (D1 and D2), the relative importance of these receptor subtypes in the development of sensitization is unknown. There are a few studies that have examined the effects of selective DA receptor antagonists o n the development of amphetamine- or morphine-induced sensitization but the results are not entirely consistent (see above); it appears likely that each receptor subtype is important. There is little evidence suggesting sensitization with subtype-selective agonists. Bromocriptine, a direct acting DA receptor agonist, is of interest in this regard for two reasons. First, this drug shows high affinity for D2 receptors, but low affinity for D1 receptors (Markstein 1981). Second, there is some evidence suggesting that locomotor activation and sensitization may occur with this compound; repeated injections of bromocriptine progressively enhance wheel-running in mice (Smith et al. 1979) and enhance circling in rats with unilateral 6-hydroxydopamine (6-OHDA) lesions of the substantia nigra (Traub et al. 1985; Hassan et al. 1986). However, the ability of bromocriptine to produce locomotor activation and sensitization, particularly in intact rats, has not been well characterized. The present experiments were designed to investigate these phenomena; a systematic analysis of the role of conditioning factors in the development of behav-
ioral sensitization was included. Characterization of bromocriptine-induced sensitization is of clinical as well as theoretical interest, since this drug is an effective therapeutic agent in the treatment of Parkinson's disease.
Experiment 1 In the first experiment the acute locomotor effects of bromocriptine were determined using standard photocell activity boxes. A 10-fold range of doses was explored, and 6-h tests were used to determine the time-course of the effects. Periodic observations of the animals were made to detect signs of stereotypic head or mouth movements that are typical of both indirect and non-selective DA agonists.
Materials and methods Subjects. Twelve male Long-Evans rats (Charles River Canada) weighing 400-500 g were housed individually in a temperaturecontrolled (21:kl °) colony room on a normal 12-h light (0800-2000)/dark cycle and served as subjects. The rats had free access to food and water. Apparatus. Activity was measured in 12 activity boxes (40.5 x 20.5 x 24.5 cm) equipped with two infrared photocell beams and detectors (at a height of 3.3 cm) which divided the length of each box into three equal areas. Each box was constructed of wood with a plexiglas front and rod floor. Ambient sounds were masked by white noise during testing. Drugs. Bromocriptine mesylate (Sandoz, Canada) was dissolved in a solution of 6% mulgofen EL-7t9 (Alkaril Chemicals Ltd, Canada) and 12% ethanol (95%) in physiological saline. The drug was injected IP 30 min prior to testing in a volume of 1 ml/kg. Procedure. The rats were tested in the activity boxes twice weekly, on Mondays and Fridays. They were habituated to the activity boxes for 3 h on the first exposure; on subsequent days, the rats were removed from their home cage and taken to a holding room where they were treated with one of four doses (2.0, 5.0, 10.0 or 20.0 mg/kg IP) of bromocriptine or its vehicle and where they remained for 30 min. The rats were then placed in the activity box located in another room and activity was recorded for 6 h. Each rat was tested with each dose and vehicle; the sequence of dosing was different for each animal.
Results and discussion
Bromocriptine had a biphasic effect on locomotor activity (Fig. 1). Following suppressed activity in the first 30 min, bromocriptine produced a dose-dependent and long-lasting increase in locomotion. A two-way analysis of variance (ANOVA) revealed a significant dose by time interaction [F(44, 484) = 5.85, P < 0.001]. During the first 30 min, each dose of bromocriptine suppressed locomotor activity relative to vehicle treatment (P,. I,-
>200 -
I--
N
300
300 " O 250 -
250 I-.
200 -
O
O 100 -
100 2:
200150 -
15o -
Z
50-
LU
ILl =E
0 30
60
90 TIME
120
150
180
(min)
500
•
,
, 30
•
,
, so
. . . . . . 90 TIME
, • 120
• , 150
,
• , 1so
(rain)
Fig. 4. Mean (± SEM) activity counts at 10-min intervals in the vehicle @[]--), paired (--II--), and unpaired groups (-a--) during the first, third, and sixth conditioning sessions. Only the paired group received bromocriptineduring activity testing
Fig. 5. Mean (+SEM) activity counts at 10-min intervals in the vehicle (--G-), paired (- B--), and unpaired groups (--zx--) during the first, third, and sixth test sessions. All groups were treated with bromocriptine during activity testing
group. The Unpaired group, which received the same number of bromocriptine injections as the Paired group but in a different environment, demonstrated an activity level similar to that observed in the Vehicle group (Fig. 5). A three-way ANOVA conducted on the first, third and sixth test sessions yielded a significant time by group interaction [F(34, 357)=6.4, PI-I j,..
loo.
O
