Self-administration of agonists selective for dopamine D2, D3, and D4 receptors by rhesus monkeys.

Dopamine receptor mechanisms are thought to play a role in the reinforcing effects of cocaine and other abused drugs. The lack of receptor-selective agonists has made it difficult to determine the role of the individual dopamine receptors in mediating these reinforcing effects. In this study, rhesus monkeys with a history of intravenous cocaine self-administration were tested for the reinforcing effects of several D3-preferring agonists, a D2-preferring agonist, and a D4 agonist. The D2-preferring agonist did not maintain responding in any monkeys, and the D4 agonist was self-administered at low rates, just above those maintained by saline in one monkey. The D3-preferring agonists were self-administered by approximately half of the animals, although at lower rates than cocaine. These results indicate the apparent limited reinforcing effectiveness of D2-like agonists requires activity at D3 receptors. Previous data from this laboratory and others also suggest that these drugs may not serve as reinforcers directly; the behavior may be maintained by response-contingent delivery of stimuli previously paired with cocaine. The ability of drug-related stimuli to maintain responding apparently differs among monkeys and other organisms, and may be related to individual differences in drug-taking behavior in humans.