The discrimination of drug mixtures using a four-choice procedure in pigeons.

RATIONALE: The purpose of these experiments was to study drug combinations as discriminative stimuli using a new four-choice procedure.

OBJECTIVES

To determine whether pigeons could discriminate among a mixture of two drugs, each of the component drugs and saline, and to study other drug combinations in these birds.

METHODS

Pigeons were trained to discriminate among saline, 5 mg/kg morphine, 5 mg/kg pentobarbital, and a mixture of these two doses using a four-choice procedure.

RESULTS

When responding stabilized, the birds responded on the correct key more than 90% of the time. Low doses of all drugs given alone produced responding on the saline key. Higher doses of pentobarbital and chlordiazepoxide produced responding on the pentobarbital key, and higher doses of morphine produced responses on the morphine key. Methamphetamine produced responding on the saline key. None of the drugs given alone produced responding on the mixture key. When pentobarbital was combined with morphine, doses both below and above the combined training doses of these drugs usually produced responding on the mixture key. The combination of chlordiazepoxide with morphine produced similar results. Combinations of methamphetamine with pentobarbital or with morphine produced effects similar to those of pentobarbital or morphine given alone.

CONCLUSIONS

A wide range of combinations of pentobarbital and morphine, or chlordiazepoxide and morphine produce responding on the mixture key, even though the pigeons were not exposed to these dose combinations during training. The four-choice procedure provides the opportunity to study drug mixtures in a detail not possible with more limited response choices.