Philip Terry

Relationships among dopamine transporter affinities and cocaine-like discriminative-stimulus effects

Abstract  Rationale: The discriminative-stimulus effects of cocaine have been reported to be mediated by indirect agonist actions initiated by the blockade of dopamine uptake, and the potencies of drugs that have discriminative-stimulus effects like cocaine are directly related to their dopamine transporter binding affinities. The binding to the dopamine transporter by cocaine and many of its analogs has been reported to fit better using a two-site model than a one-site model. Objectives: The present study examined the relationship among binding affinities of dopamine uptake inhibitors at these two sites and their potencies to produce discriminative-stimulus effects. Methods: The inhibition constants (Ki values) were derived for unlabeled dopamine uptake inhibitors for displacement of [3H]WIN 35,428 from rat caudate putamen membranes. These Ki values were related to the ED50 values obtained in rats trained to discriminate 10 mg/kg cocaine from saline injections under a fixed-ratio 20 schedule of food reinforcement. Results: Among the dopamine uptake inhibitors studied, the binding data for eight compounds (WIN 35,428, nomifensine, WIN 35,981, WIN 35,065-2, methylphenidate, cocaine, cocaethylene, and bupropion) were better fit by a two-site model than a one-site model. The data for the remaining eleven compounds (RTI-31, RTI-55, RTI-121, RTI-32, LU19-005, BTCP, GBR12909, GBR12935, mazindol, LU17-133, and EXP561) were better fit by a one-site model. Of the drugs that were fit best by a two-site model, there was a higher correlation among the Ki values for the high-affinity site and the ED50 values (R2=0.655; P=0.015) than there was for the low-affinity site (R2=0.543; P=0.037). Of the remaining drugs, there was a high correlation among the Ki values and the ED50 values for the discriminative-stimulus effects (R2=0.523; P=0.012). Conclusions: These data suggest that the discriminative-stimulus effects of cocaine are more closely related to actions mediated by high-affinity binding to the dopamine transporter than they are to actions mediated by the low-affinity site. The further assessment of the respective contributions of high- and low-affinity binding to the behavioral effects of cocaine will be greatly enhanced with the development of pharmacological tools that have a high degree of selectivity for one of these components.

Differential relationships among dopamine transporter affinities and stimulant potencies of various uptake inhibitors

Binding to the dopamine transporter and inhibiting dopamine reuptake are considered important factors in regulating behavioral effects of cocaine. One prominent behavioral effect of cocaine and other dopamine uptake inhibitors is the stimulation of locomotor activity. To examine the relationship between action at the dopamine transporter and behavior, the displacement of [3H]WIN 35,428 (CFT naphthalene sulfate; 2-beta-carbomethoxy-3-beta-(4-fluorophenyl)tropane-1,5-naphthalene disulfonate) binding in rat caudate putamen by cocaine and other uptake inhibitors was compared with stimulation of mouse locomotor activity. There was a significant correlation among affinities for binding and potencies for stimulating activity for cocaine and structurally similar compounds. For structurally dissimilar uptake inhibitors, however, there was no significant correlation among potencies for stimulation of activity and affinity for displacement of [3H]WIN 35,428 binding. These findings provide evidence that cocaine analogs may bind to the dopamine transporter in a manner that is fundamentally different from that for structurally dissimilar uptake inhibitors.

Asymmetric generalization between the discriminative stimulus effects of nicotine and cocaine.

The discriminative stimulus effects of nicotine and cocaine were studied, alone and in combination, in rats. Two sets of rats were trained to press one lever when injected intraperitoneally (i.p.) with either nicotine (0.1 mg/kg = 0.6 micromol/kg, Set 1) or cocaine (8.9 mg/kg base = 29.4 micromol/kg, Set 2), and another lever when injected with saline. Rats learned to discriminate drug from saline, and maintained discriminative control throughout the study (at > 85% drug-appropriate responding). In accordance with most previous findings, cocaine only partially substituted for nicotine (maximum = 41% nicotine-lever responding). The nicotinic agonist, nornicotine, produced dose-related, near-full substitution for nicotine (maximum = 76% nicotine-lever responding), whereas the peripherally acting nicotinic agonist, methylcarbamylcholine, did not substitute for nicotine. The muscarinic receptor agonist pilocarpine also failed to substitute for nicotine. However, in the cocaine-trained rats, nicotine substituted fully for cocaine in a dose-dependent manner, demonstrating that cross-generalization between the two drugs is not symmetrical. Finally, administration of each drug as a pre-treatment to the other yielded inconsistent increases in each drugs discriminative stimulus effects. The results are congruent with the view that the discriminative stimulus effects of nicotine and cocaine share common features, but the asymmetric pattern of cross-generalization and the interactions revealed in the combination tests also suggest that there are important differences between them.

A comparison of the locomotor stimulant effects of D1-like receptor agonists in mice

Efficacy in stimulating adenylyl cyclase (AC) has traditionally been used to distinguish dopamine D1-like receptor agonists from dopamine D2-like receptor agonists. However, there is a limited association between the effects of D1-like agonists in behavioral assays and their effectiveness at stimulating AC. Other second messenger actions might contribute to the behavioral effects of D1-like agonists, as there is evidence for a link to the hydrolysis of phosphoinositide (PI). The present study compared the locomotor stimulant effects of five D1-like receptor agonists having different efficacies in assays of AC and PI activity. All D1-like agonists produced long-lasting biphasic effects on locomotor activity. SKF 38393, the prototypical partial agonist (based on AC activity), produced limited changes in locomotor activity, whereas the partial agonists SKF 75670 and SKF 77434 produced locomotor stimulant effects that were similar to or greater than those of the full efficacy agonists SKF 82958 and SKF 81297. However, there did not appear to be a relationship between maximal behavioral effects and AC stimulation or PI hydrolysis. The results suggest a complex relationship between the behavioral effects of D1-like agonists and their intrinsic efficacies as measured by AC and /or PI stimulation. Although a limited number of compounds were examined, neither second messenger system alone appears to account fully for these behavioral effects. The current classification of D1-like agonists according to their intrinsic efficacies as defined by AC stimulation needs further scrutiny.

Differential effects of two cannabinoid receptor agonists on progressive ratio responding for food and free-feeding in rats

The cannabinoid receptor agonists Δ9-tetrahydrocannabinol (Δ9-THC) and HU-210 were compared in terms of their effects on: (1) progressive ratio (PR) responding for food, and (2) free food intake. In the first experiment, food-deprived Wistar rats were trained on a time-constrained (60 min) PR-5 schedule for food reinforcement, in which the response requirement incremented by five lever presses for each successive reinforcer. One group of rats received vehicle, 0.5, 1 or 3 mg/kg Δ9-THC (i.p.), and three other groups received HU-210 (i.p.) at three different dose ranges, spanning 0.001−0.1 mg/kg. In the second experiment, the effects of the two drugs on free food intake were tested in a separate group of non-deprived rats. For PR responding, Δ9-THC significantly increased the break point (final ratio completed) and the total number of lever presses emitted. The same drug also significantly increased free food intake. However, the effects of HU-210 were quite different: it did not alter PR responding at any dose; instead, its only significant effect was to reduce free food intake at 0.06 mg/kg. These data suggest that increased motivation to obtain food might underlie the hyperphagic effects of Δ9-THC. However, the synthetic agonist HU-210 has different effects: it only acts to reduce feeding behaviour, an outcome that probably reflects non-specific behavioural disruption. These findings suggest important differences between the two CB1 receptor agonists in terms of their pharmacological effects.

Factors contributing to changes in frequency of cannabis consumption by cannabis users in England: A structured interview study

Structured interviews were conducted to identify factors that contribute to changes in frequency of personal cannabis use (increases, decreases or abstinences). Sixty cannabis users were recruited from locations throughout England (mean duration of use: 14.1 years). Among those who gave reasons for escalating their cannabis use, a large majority (79%) cited the importance of increased use by others. Approximately one-third cited the importance of a change in circumstances that provided more opportunities to use the drug, and a similar proportion claimed a role for the perceived beneficial effects of the drug. The reasons given for reducing or abstaining were more diverse, but changed circumstances were again highlighted (by 55% and 73% respectively). Reductions primarily followed changes to living arrangements, peer relations or employment; in contrast, changes precipitating abstention were more transient. Negative health effects were considered relevant by 31% of reducers and by the same proportion of those who had ever abstained. The results suggest that perceived external constraints play important roles in both upward and downward changes in cannabis use; that perceived functional effects influence escalation; and that concerns about addiction and health are salient to reduced use.

Differential effects of injection regimen on behavioral responses to cocaine

Locomotor behavior was measured in mice receiving IP cocaine at 5, 10, 20, or 40 mg/kg. then with 5, 5, 10, and 20 mg/kg at 10-min intervals. In the single-dose treatment, mice received a single dose of cocaine at a time corresponding to the equivalent cumulative dose, with saline injections at other times. The single-injection treatment was similar, but saline injections were omitted. Locomotor activity was measured across each 10-min interval. Mice were retested 6 weeks later and 1 day after that. Dose-response curves were similar for all three treatments on the first test, but diverged markedly on subsequent tests. Significant locomotor sensitization occurred at the higher doses on the second test, particularly in the treatments receiving single cocaine injections. On the third test, convulsions occurred at 40 mg/kg, but only in the singly dosed treatments. The results demonstrate that injection parameters can modify both the behavioral and toxic effects of cocaine.

Acute and chronic effects of propranolol on extinction of rewarded running in the rat.

Three experiments examined the effect of propranolol injected IP on a series of 16 extinction trials at 24-hour intertrial intervals after continuously rewarded running. In Experiment I, propranolol (1, 5 or 10 mg/kg) was injected, in different groups, shortly before or shortly after each extinction trial, but without effect. In Experiment II, the same doses were injected daily for 18 days preceding extinction, to allow longer for any long-term drug effect to accumulate. Propranolol (1 mg/kg) facilitated extinction. In Experiment III, a single dose (7.5 mg/kg) was injected before or after extinction trials as in Experiment I; the acute effect was to impair extinction. These results are consistent with previous findings in the Skinner box that propranolol has an acute disinhibitory effect on nonrewarded responses and a long-term inhibitory one. These effects have different dose-response relationships.

Conditioned grooming induced by the dopamine D1-like receptor agonist SKF 38393 in rats.

Stimulation of dopamine D1-like receptors reliably increases grooming in rats and mice. The study examined whether the grooming response elicited by the prototypical D1-like agonist SKF 38393 (8 mg/kg s.c.) could be conditioned to the specific environment in which it occurred. Rats in one group (Paired) received SKF 38393 and rats in another group (Unpaired) received saline in observation boxes outside of their housing room; the rats were then scored for duration and frequency of grooming bouts over 25 min. The ordering of injections was reversed the next day in the rats, housing room. The procedure was repeated twice, with at least one intervening drug-free day, to give three conditioning trials. The D1-like agonist significantly increased grooming on each of the three conditioning trials, without obvious tolerance or sensitization, and the effect tended to persist for the duration of each trial. On the test trial for conditioned grooming, mean grooming duration was significantly greater in the Paired than the Unpaired group, suggesting that SKF 38393-induced grooming had been conditioned to the test environment. This is the first time that drug-elicited grooming has been conditioned to environmental cues.

The effects of alcohol on the recognition of facial expressions and microexpressions of emotion: enhanced recognition of disgust and contempt

The study compared alcohols effects on the recognition of briefly displayed facial expressions of emotion (so‐called microexpressions) with expressions presented for a longer period.