Abstract
Dopamine D1 ligands have been classified and ordered according to efficacy in both in vitro and in vivo studies. In the present experiments, dopamine D1 ligands reported to differ in in vitro efficacy were evaluated for efficacy-related effects on eye blinking in squirrel monkeys. Additional comparisons were made with the effects of D2 receptor agonists and indirect dopamine agonists. The results show that D1 agonists increased eye blinking in an efficacy-related manner, whereas the D1 receptor blocker SCH 39166 [(–)-trans-6,7,7α,8,9,13β-hexahydro-3-chloro-2-hydroxy-N-methyl-5H-benzo[d]naphtho[2,1-b]azepine] only decreased rates of eye blinking. D1 high-efficacy agonists induced rates of eye blinking that were 2- to 3-fold greater than observed with dopamine D2 agonists and indirect agonists. In drug combination experiments, increases in eye blinking induced by the D1 high-efficacy agonist R-(+)-6-Br-APB [R-(+)-6-bromo-7,8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrobromide] were antagonized by both the D1 antagonist SCH 39166 and the lower efficacy agonist SKF 83959 [6-chloro-7,8-dihydroxy-3-methyl-1-(3-methylphenyl)-2,3,4,5-tetrahydro-1H-3-benzazepine hydrobromide], consistent with dopamine D1 receptor mediation of these behavioral effects. The dopamine D2 agonist (+)-PHNO [(+)-N-propyl-hydroxynaphthoxazine], which selectively activates dopamine D2 receptors, also attenuated D1 agonist-induced increase in eye blinking, suggesting that D2 receptor actions may inhibit D1-mediated increases in eye blinking. Overall, eye blink rate appears to be a robust behavioral measure that can be used to measure changes in dopaminergic D1 signaling and as a functional assay of agonist efficacy at dopamine D1 receptors.
Dopamine D1 receptor agonists have been ordered according to drug efficacy in both in vitro and in vivo experiments (for summaries of current literature, see Tables 1 and 2). In vitro analyses most often have been based on the extent to which D1 ligands activate adenylyl cyclase (Kebabian and Calne, 1979) or, more recently, phospholipase C-mediated phosphoinositide (PI) hydrolysis in brain tissue (Undie et al., 1994). Although both assays have permitted the identification of differing levels of drug efficacy, it is important to note that results have differed across species or experimental conditions and that correspondence between in vitro and in vivo efficacy estimates for D1 ligands is not uniformly evident. For example, both the D1 agonists SKF 38393 and SKF 81297 stimulate adenylyl cyclase with low to moderate efficacy in both native striatal and cultured astrocytic tissue from primate species (Vermeulen et al., 1994). In rat brain, however, SKF 38393 continues to display low to moderate D1 efficacy whereas SKF 81297 produces high, dopamine-like levels of cyclase stimulation (Andersen and Jansen, 1990; Pifl et al., 1991; Izenwasser and Katz, 1993; Watts et al., 1993). Also, the D1 agonist SKF 83959, which has clear agonist effects in MPTP-lesioned rats and monkeys, fails to stimulate adenylyl cyclase in striatal tissue from untreated rats, resulting in its characterization as an “atypical” D1 agonist (Gnanalingham et al., 1995; Waddington et al., 1995). However, like SKF 38393, SKF 83959 has good efficacy in stimulating PI turnover, raising the possibility that the two in vitro assays may be associated with different physiological or behavioral endpoints (Panchalingam and Undie, 2001; Jin et al., 2003).
Across in vivo evaluations of dopamine ligands, efficacy-related differences in D1-mediated behavioral effects have been more readily observed in mice and monkeys than in rats (see Bergman et al., 2000). Behavioral studies typically indicate that high-efficacy D1 agonists engender more observable behavior than lower efficacy agonists. For example, Arnt et al. (1992) reported that D1 agonists with high efficacy for stimulating cyclase activity also produced high levels of locomotion in mice, whereas lesser levels were observed with lower efficacy agonists including SKF 83959 and SKF 75670. In MPTP-induced hemiparkinsonian monkeys, high-efficacy agonists including A-77636 and SKF 82958 induced contraversive turning, an effect not observed with the D1 intermediate-efficacy agonist SKF 81297 (Domino and Sheng, 1993). Additionally, D1 low-efficacy agonists, but not high-efficacy agonists, have been shown to produce catalepsy-associated behavior in squirrel monkeys (Rosenzweig-Lipson and Bergman, 1994). Although an efficacy relationship among drugs was not discerned in those studies, it is noteworthy that in vitro stimulation of adenylate cyclase activity or PI turnover does not preclude the production of in vivo antagonist-like effects.
Efficacy-related effects of D1 agonists in monkeys also have been reported in studies of operant performance. For example, the extent to which the D1 receptor blocker SCH 39166 surmountably antagonized the rate-decreasing effects of agonists in studies of schedule-controlled responding differed among selective D1 ligands in an efficacy-related manner (Bergman et al., 1995). Additionally, compounds considered to have intermediate to high efficacy, e.g., SKF 81297, SKF 82958, and R-(+)-6-Br-APB, were found to support i.v. self-administration behavior in monkeys and to produce cocaine or methamphetamine-like discriminative stimulus effects. Lower efficacy agonists, e.g., SKF 38393 and SKF 77434, were not self-administered and antagonized effects of the indirect dopamine agonists in drug-discrimination experiments (Spealman et al., 1991a; Weed and Woolverton, 1995; Grech et al., 1996; Spealman et al., 1997; Weed et al., 1997; Tidey and Bergman, 1998; Sinnott and Nader, 2001). Taken together, differences in the effects of D1 ligands in these several studies of operant performance have pointed to a functional separation between drugs with intermediate to high efficacy and those with low to no efficacy in in vitro studies of cyclase stimulation (Weed and Woolverton, 1997; Bergman et al., 2000; Tables 1 and 2).
The induction of eye blinking is another behavioral effect of dopaminergic agonists that may be amenable to efficacy-based evaluations. Spontaneous eye blinking is associated with central dopaminergic activity, and as documented in dopamine-related pathologies in humans, the disruption of dopamine signaling can alter basal blink rates. For example, parkinsonian patients have decreased dopamine levels and demonstrate decreased blink rates compared with healthy subjects (Karson, 1983; Deuschl and Goddemeier, 1998). In contrast, schizophrenic patients, typically considered to have elevated dopamine levels, have elevated blink rates that can be normalized by treatment with dopamine receptor-blocking neuroleptics (Karson et al., 1981; Karson, 1983). Altered blink rates also have been measured in other dopamine-associated central nervous system disorders such as attention-deficit-hyperactivity disorder (Caplan et al., 1996), anxiety and panic disorders (Kojima et al., 2002), and depression (Ebert et al., 1996). However, the extent to which such irregularities involve disruptions in dopaminergic signaling will remain uncertain until the mechanistic bases for these pathologies are better understood.
Consistent with findings in human patients, eye blinking also is reduced in animal models of Parkinson's disease. For example, the severity of parkinsonian symptoms in MPTP-treated African green monkeys correlated well with the extent to which blink rates decreased in these subjects (Lawrence and Redmond, 1991). Despite clear evidence for dopaminergic involvement in eye blinking, however, few studies have used this behavioral endpoint to investigate the pharmacology of dopamine agonists. Both dopamine D1 and D2 receptor agonists can increase eye blinking in vervets and cynomolgus monkeys (Elsworth et al., 1991; Kleven and Koek, 1996), and of interest, a lower efficacy D1 agonist, SKF 75760, appeared not to increase eye blinking to the same extent as high efficacy D1 agonists in cynomolgus monkeys (Kleven and Koek, 1996). Although suggestive, these initial studies have not been pursued more fully to evaluate the relationship between ligand efficacy and blink rates.
The current research was designed to address this issue by evaluating the effects of dopamine-related ligands on eye blink rates in squirrel monkeys. Squirrel monkeys were used because previous studies provide comparative information regarding other behavioral effects of dopaminergic drugs in this species. First, experiments were conducted to document the effects of D1 ligands ranging in efficacy from full agonist to antagonist on eye blink rates. The effects of the D2 agonist (+)-PHNO and the indirect dopamine agonist methamphetamine also were determined. Next, the antagonistic effects of the D1 receptor blocker SCH 39166 and the D1 low-efficacy agonist SKF 83959 were compared in monkeys treated with the high-efficacy D1 agonist R-(+)-6-Br-APB. Finally, drug interaction experiments were conducted to determine how D1-mediated alterations in eye blink rate were modified by the selective D2 agonist (+)-PHNO and D2 receptor blocker haloperidol. The results of these studies show that: D1 agonists increase eye blinking in a dose-dependent manner and that the magnitude of increase is related to drug efficacy; D1 antagonists and D1 low-efficacy agonists produce rightward shifts in the dose effect curves of D1 high-efficacy agonists, indicative of D1 receptor mediation; and cotreatment with the selective D2 agonist (+)-PHNO appeared to block D1 agonist-induced stimulation of eye blinking, suggesting that D2 receptor activation may inhibit such D1 receptor-mediated effects.
Materials and Methods
Subjects. A group of 12 squirrel monkeys (Saimiri sciureus), weighing 750 to 950 g, were housed individually in a climate-controlled vivarium. Monkeys had unlimited access to water and received a daily allotment of high-protein monkey chow (Purina Monkey Chow; Purina, St. Louis, MO), supplemented with fruit and multivitamins. Some monkeys (Ss323, Ss484, Ss60, Ss98, Ss152, and Ss62) had previously participated in behavioral experiments and received dopamine agonists and antagonists. Monkeys were weighed daily, and their diets were adjusted regularly to maintain constant body weights. The protocol for animal use in these studies was approved by the Institutional Animal Care and Use Committee at McLean Hospital. Subjects in this study were maintained in accordance with guidelines provided by the Committee on Care and Use of Laboratory Animals of the Institute of Laboratory Animals Resources, National Institutes of Health. This facility is licensed by the U.S. Department of Agriculture.
Apparatus and Experimental Methods: Observational Procedures. Eye blinking was measured during observational experiments that were conducted in a specially constructed chamber. Monkeys sat in a chair with blackened side and back walls to provide color contrast. The front wall of the chair was removed to facilitate videotaping, and a neck plate that maintained a relatively constant orientation of the head was used to facilitate observation of eye blink responses. A curved piece of mirrored Plexiglas also was placed behind the monkey's head to permit viewing of the eye blink response when the monkey turned its head. A compact video camera (JVC, model GR-AX10), equipped with a telephoto lens, was positioned at a distance of approximately 2 feet in front of the seated monkey. Two vertical fluorescent lights on the side walls of the chamber were positioned to provide optimum illumination. The camera image was captured by computer (ATI Multimedia Software, ATI Player) for viewing and recording onto videotape.
In daily control sessions, each monkey was studied for four or five consecutive 15-min components, each consisting of a 10-min habituation period followed by a 5-min period during which eye blinking was videotaped. Test sessions were also comprised of four or five consecutive components. The video camera displayed the image on a computer screen during the entire video session to allow continual observation of the subject. Observers blind to the treatment conditions later scored the 5-min videotaped session components. An eye blink was defined as a visible, rapid opening and closing of the eyelid. Test components periodically were rescored by other trained observers to confirm measurement reliability > 90%.
Drug Testing. The direct effects of drugs on eye blinking were studied once or twice a week using cumulative dosing procedures. Briefly, incremental doses of a drug were administered at the outset of the 15-min components of the test session, and the effects of a maximum of five cumulative doses could be determined during a single experiment. For most drugs, a complete dose effect curve was established in one session; however, for some drugs, a full dose effect curve could only be obtained by testing two or three overlapping dose ranges. Each drug was studied in four or more subjects, and the order of drug testing varied among individuals.
Drug combination studies were conducted in groups of three or four monkeys (see Table 3). For antagonism studies, each of several doses of either SCH 39166 (0.1–1.0 mg/kg) or SKF 83959 (0.1–3.0 mg/kg) was administered i.m. 15 min before test sessions in which the effects of cumulative doses of R-(+)-6-Br-APB were determined in successive components. In experiments to evaluate D1/D2 interactions, each of several i.m. doses of haloperidol (0.1–1.0 mg/kg) or a single dose of (+)-PHNO, 0.003 mg/kg, was administered 15 min before test sessions in which the effects of cumulative doses of SKF 82958 were determined. Observations and videotaping were conducted in the manner previously described. For time course studies, a single injection of SKF 82958 (0.1 or 1.0 mg/kg) was administered i.m. prior to videotaping 5-min periods at 5, 10, 15, 30, 45, 60, and 120 min postinjection.
Drugs. Methamphetamine, SKF 82958, SKF 38393, SKF 77434, SKF 81297, and R-(+)-6-Br-APB were purchased from Research Biochemicals International Sigma/RBI (Natick, MA). SKF 83959 was supplied by Sigma/RBI through the Chemical Synthesis Program of the National Institute of Mental Health (Contract N01MH30003). SCH 39166 and (+)-PHNO were graciously supplied by Schering Plough (Kenilworth, NJ) and Merck Sharp and Dohme (Hoddesdon, UK), respectively. Except for the benzazepines, all drugs were prepared in sterile water and warmed and/or sonicated prior to administration as necessary. Stock solutions of the benzazepines were prepared in a solution of 0.1% ascorbic acid in sterile water prior to further dilution. All drugs were administered by i.m. injection in volumes of approximately 0.2 to 0.4 ml.
Data Analysis. Eye blink rates were computed as total number of eye blinks divided by the duration of time that the subject's eyes were visible during the 5-min test component (usually >290 s). Vehicle injections did not affect rates of eye blinking during the four sequential components of vehicle control sessions; therefore, mean control values for rates of eye blinking (mean ± S.E.M.) were calculated by averaging blink rates across vehicle control sessions. For individual monkeys, data were expressed as the rate of eye blinking effect after each cumulative dose (blinks per minute). The effects of vehicle and each drug were averaged for each group of monkeys to express mean ± S.E.M. values. The effects of a dose of drug were considered to be statistically significant when the mean value for that dose lay outside the 99% confidence interval for mean control rates of eye blinking in the same group of monkeys.
Results
Control Rates of Eye Blinking. Control values for eye blink rates never exceeded 20 blinks/min in any subject and, for the group of 12 monkeys, averaged 9.2 ± 1.4 blinks/min. Control values for different groups of subjects in which drugs were studied ranged from 5.5 ± 0.3 to 13.2 ± 2.2 blinks/min. Table 3 shows that control values varied somewhat across monkeys but were relatively stable across control sessions within individual subjects. Factors that contributed to individual differences in blink rate were not identified, and monkeys were not grouped for drug testing on the basis of control values. However, monkeys with higher blink rates were fully mature and had participated in extended studies of the behavioral effects of dopaminergic ligands, whereas monkeys with lower blink rates were relatively young and had not previously participated in such studies.
Effects of Dopamine D1 Ligands. The dopamine D1 agonist R-(+)-6-Br-APB produced the highest rates of eye blinking, demonstrating a 9-fold increase over control values at the highest dose tested, 0.3 mg/kg (Fig. 1a). The dopamine D1 agonists SKF 81297 and SKF 82958 were comparably potent and produced high rates of eye blinking, yielding 7- to 8-fold increases over control values, respectively (Fig. 1a). A time course evaluation of the highest dose of SKF 82958, 1.0 mg/kg, showed that rates of eye blinking increased with a rapid onset of action that dissipated in an exponential manner over the course of 60 min (Fig. 2). The lower dose of SKF 82958, 0.1 mg/kg, produced small increases in rates of eye blinking initially after injection, and these elevated rates completely returned to baseline values by 45 min.
The D1 agonists SKF 83959 and SKF 77434 also generally increased rates of eye blinking, although to a lesser extent than observed with SKF 82958, SKF 81297, and R-(+)-6-Br-APB (Fig. 1b). Cumulative doses of SKF 83959 resulted in rates of eye blinking that averaged approximately 4-fold above control values and appeared to plateau at this level. SKF 77434, at 3.0 and 10.0 mg/kg, also increased rates of eye blinking 3.5-fold above control values. The higher dose of 17.8 mg/kg SKF 77434 either did not further increase eye blink rates or, in some monkeys, decreased blink rates toward control values. The agonist SKF 38393 produced small, if any, increases in rates of eye blinking (Fig. 1b). The largest effect observed with SKF 38393 was a 1.5-fold increase over control values with the lowest dose tested (3.0 mg/kg). The highest dose of SKF 38393, 30 mg/kg, decreased rates of eye blinking to 50% of control values.
In contrast to the increases in rates of eye blinking produced by most D1 agonists, the D1 receptor blocker SCH 39166 only decreased rates of eye blinking (Fig. 1b). Low doses, 0.01 to 0.03 mg/kg, slightly decreased blink rates to 84 and 77% of control values, respectively. The larger dose of 0.1 mg/kg SCH 39166 decreased rates of eye blinking to an average of 2 blinks/min, or approximately 12% of control blink rates.
The maximum increase in eye blink rates is plotted for each dopamine D1 ligand in Fig. 3. As shown, R-(+)-6-Br-APB produced the largest increase above vehicle, followed by SKF 82958, SKF 81297, SKF 83959, and SKF 77434. The remaining two ligands SKF 38393 and SCH 39166 did not stimulate eye blink rates significantly above vehicle values.
Effects of Indirect and D2 Dopamine Agonists. The indirect dopamine agonist methamphetamine produced dose-dependent increases in rates of eye blinking (Fig. 1c). At a dose of 1.0 mg/kg, methamphetamine increased blink rate to 4-fold above control values. A higher dose of methamphetamine, 3.2 mg/kg, did not appear to increase eye blink rates further; however, this higher dose of methamphetamine produced agitation and other behavioral effects that precluded accurate scoring of blink rates and, therefore, was tested in one subject only. The dopamine D2 agonists, quinelorane and (+)-PHNO, produced small increases in rates of eye to approximately 20 blinks/min (Fig. 1c).
Drug Interaction Studies. Several doses of SCH 39166 were administered prior to cumulative dose effect determinations of R-(+)-6-Br-APB-induced eye blinking. SCH 39166 antagonized the effects of the D1 agonist R-(+)-6-Br-APB on eye blinking and shifted the dose effect function rightward in a parallel manner (Fig. 4). Each half-log unit increase in the dose of SCH 39166 shifted the R-(+)-6-Br-APB dose effect curve approximately 3-fold further to the right. A Schild plot analysis of the antagonist effects of SCH 39166 revealed a pA2 value of 7.12 with a slope of –1.22 ± 0.23.
In further experiments, SKF 83959 also was administered prior to R-(+)-6-Br-APB. Like SCH 39166, SKF 83959 generally shifted the dose effect curve for R-(+)-6-Br-APB-induced eye blinking to the right (Fig. 5). Pretreatment with the low dose of SKF 83959, 0.1 mg/kg, did not shift the entire dose effect curve but attenuated the effects of the highest dose of R-(+)-6-Br-APB. The intermediate doses of SKF 83959, 0.3 and 1.0 mg/kg, produced approximate 3-fold shifts in the R-(+)-6-Br-APB dose effect curve, and the highest dose, 3.0 mg/kg SKF 83959, shifted the curve approximately 30-fold to the right.
The dopamine D2 antagonist haloperidol was administered prior to cumulative doses of the D1 full agonist SKF 82958 and did not appear to antagonize the effects of SKF 82958 in a surmountable manner (Fig. 6). Thus, whereas relatively high doses of haloperidol may have partially blocked the effects of 1.0 mg/kg SKF 82958, the effects of haloperidol were neither dose-related nor robust (≤3-fold change following the high dose of 1.0 mg/kg).
The dopamine D2 agonist (+)-PHNO was administered alone or prior to cumulative doses of the D1 full agonist SKF 82958 (Fig. 7). The dose of 0.003 mg/kg (+)-PHNO alone increased eye blinking to 27 blinks/min. When administered as a pretreatment to SKF 82958, 0.003 mg/kg (+)-PHNO increased eye blinking produced by a low dose of SKF 82958 alone (0.03 mg/kg) and decreased eye blinking induced by higher doses of SKF 82958 alone (0.1 and 1.0 mg/kg).
Discussion
R-(+)-6-Br-APB, SKF 82958, and SKF 81297 were the most efficacious compounds, leading to rates of eye blinking that were 7- to 9-fold above control values. SKF 82958 and SKF 81297 were comparably potent and of slightly lesser potency than R-(+)-6-Br-APB. SKF 77434 and SKF 83959 demonstrated lower efficacy, increasing eye blinking only 3-fold above control values. Although these D1 partial agonists produced similar increases in eye blinking, SKF 83959 was 100 times more potent than SKF 77434. The highly selective dopamine D1 ligand SKF 38393 was a weak agonist in these studies, producing very small increases in eye blinking at the doses tested. Using eye blinking measurements, the rank order of efficacy for these dopamine D1 agonists R-(+)-6-Br-APB > SKF 82958 ≥ SKF 81297 > SKF 83959 = SKF 77434 > SKF 38393. These data are consistent with some previous efficacy classifications of dopamine D1 agonists. Interestingly, previous work demonstrated that SKF 81297 has low efficacy for increasing adenylate cyclase activity in squirrel monkey striatal tissue (Izenwasser and Katz, 1993); nevertheless, this compound produces behavioral effects similar to those of high-efficacy agonists in the present study as well as in other behavioral studies in monkeys (Spealman et al., 1997; Weed et al., 1997; Tidey and Bergman, 1998). Overall, these data demonstrate that D1 agonists can elicit a high degree of eye blinking activity. The large range of effects that can be observed suggests that measurement of eye blinking can be a useful indicator of dopamine D1 agonist efficacy in primate species.
The effects of D1 agonists on eye blinking appear to be mediated solely by D1 receptor activation. The selective dopamine D1 antagonist SCH 39166 produced parallel shifts in the R-(+)-6-Br-APB dose effect curve, consistent with mediation by the dopamine D1 receptor. In this regard, pA2 analysis of the shift in the R-(+)-6-Br-APB dose effect curve revealed a slope of 1.22 (±0.23) and a pA2 value of 7.12, supporting the view of competitive antagonism at the D1 dopamine receptor. Moreover, the D2 dopamine receptor antagonist haloperidol did not reliably antagonize the effects of the D1 agonist SKF 82958, consistent with a predominant role for the D1 receptor in D1 agonist-induced changes in eye blinking.
The D1 low-efficacy agonist SKF 83959 produced moderate that plateaued at an intermediate level increases in eye blinking and produced rightward shifts in the dose effect curve for the D1 full agonist R-(+)-6-Br-APB. These data suggest that SKF 83959 is a partial agonist at the dopamine D1 receptor, and as with the behavioral effects of other pharmacological classes, that partial agonists in the D1 receptor system can act as competitive antagonists.
Dopamine D2 receptor activation with the D2 agonists, (+)-PHNO and quinelorane, only produced small increases in eye blinking. However, D2 receptor activation with (+)-PHNO also attenuated D1 agonist-induced increases in eye blinking. Based on previous findings, it is unlikely that (+)-PHNO is activating the D1 receptor at the doses used in this study (Elsworth et al., 1991; Domino and Sheng, 1993; Rosenzweig-Lipson et al., 1994; Bergman et al., 1995). More likely, D2 receptor activation by (+)-PHNO may have inhibited D1 agonist-induced eye blinking in these experiments. D1/D2 agonist interactions have received some attention in previous behavioral studies. In keeping with earlier suggestions of facilitative or synergistic interactive effects of D1 and D2 receptor activation (Walters et al., 1987), coadministration of D1 and D2 agonists has been reported to produce exaggerated contralateral turning in 6-OHDA-lesioned rats (Gnanalingham et al., 1995). However, in the same study, this synergistic effect was not observed in MPTP-treated monkeys. Likewise, combined activation of D1 and D2 receptors has not been found to potentiate the effects of either compound in either methamphetamine discrimination studies in rats (Nielsen et al., 1989) or cocaine discrimination studies in monkeys (Spealman et al., 1991b). The present findings provide further evidence that D1 and D2 receptor agonism is not necessarily additive or synergistic and, moreover, that D2 receptor activation can moderate behavioral effects of dopamine D1 receptor activation. In this regard, it may be noteworthy that, in the present study, the indirect dopamine agonist methamphetamine only increased eye blinking 4- to 6-fold above control values. Possibly, the limited effects of methamphetamine on eye blinking are indicative of moderating effects resulting from its indirect activation of dopamine D2 receptors.
Interestingly, the D1 antagonist SCH 39166 nearly eliminated eye blinking (88% decrease) at the highest doses tested. The state of dopamine D1 receptors in vivo is not yet fully elucidated, leaving the mechanism that mediates these effects open to speculation. It may be that dopaminergic tone regulates normal levels of eye blinking and, thus, that blocking D1 receptors decreases dopamine-mediated signaling and inhibits eye blinking. Similar explanations can be used to explain decreased eye blinking in human Parkinson's patients, in which decreased dopaminergic activity has been related to decreases in eye blink rate. On the other hand, the dopamine receptor may be constitutively active. In this case, SCH 39166 may have inverse agonist activity that could functionally decrease D1 receptor-G protein coupling (Cai et al., 1999; Martin et al., 2001), leading to the same behavioral outcome. From this perspective, compounds that previously have been classified as neutral antagonists but have inverse agonist activity would be anticipated to have direct effects, in addition to their antagonist action. However, it should be noted that dopamine D1 antagonists including SCH 39166 can have behavioral effects comparable to those of low-efficacy D1 agonists (Rosenzweig-Lipson and Bergman, 1994). These observations may be more consistent with drug-induced decreases in D1 receptor activation than inverse agonist activity produced by D1 receptor blockers.
In conclusion, the present data demonstrate that eye blink measurement can be used to identify dopamine D1 agonists differing in agonist efficacy and to distinguish between different types (i.e., D1, D2) of dopaminergic actions. Additionally, the present studies provide an interesting insight into D1/D2 receptor interactions, suggesting that D2 receptor activation may moderate some D1 agonist-induced behavioral effects in a primate species. These data provide a basis for using eye blinking as an assay with which to identify and classify novel dopamine D1 agonists as well as to further investigate the behavioral consequences of dopamine D1 and D2 receptor-mediated interactions.
Acknowledgments
We thank N. Mutschler and H. Millette for assistance in conducting these studies.
Footnotes
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This work was supported by National Institutes of Health Grants DA10566, DA11453, and DA03774.
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doi:10.1124/jpet.104.071092.
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ABBREVIATIONS: PI, phosphoinositide; SKF 38393, (±)-1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol hydrochloride; SKF 81297, (±)-6-chloro-7,8-dihydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrobromide; SKF 83959, 6-chloro-7,8-dihydroxy-3-methyl-1-(3-methylphenyl)-2,3,4,5-tetrahydro-1H-3-benzazepine hydrobromide; MPTP, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine; SKF 82958, (±)-6-chloro-7,8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrobromide; SCH 39166, (–)-trans-6,7,7α,8,9,13β-hexahydro-3-chloro-2-hydroxy-N-methyl-5H-benzo[d]naphtho[2,1-b]azepine; R-(+)-6-Br-APB, R-(+)-6-bromo-7,8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrobromide; SKF 77434, (±)-7,8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrobromide; A-77636, (1R,3S)-3-(1′-adamantyl)-1-aminomethyl-3,4-dihydro-5,6-dihydroxy-1H-2-benzopyran; (+)-PHNO, (+)-N-propyl-hydroxynaphthoxazine; 6-OHDA, 6-hydroxydopamine; CI, confidence interval; SKF 75670, R,S-7,8-dihydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine.
- Received May 7, 2004.
- Accepted July 30, 2004.
- The American Society for Pharmacology and Experimental Therapeutics