Hypothermic Effects of Dopamine D₃ Receptor Agonists in the Island of Calleja Magna. Potentiation by D₁ Activation

Abstract

The selective functions of D₃ receptors in the brain are still poorly understood, mainly because all the ligands active at dopamine D₃ receptors have also a high affinity for the D₂ receptors. However, it is possible to study selectively D₃ receptor function because some brain structures, such as the islands of Calleja, contain D₃ and not D₂ receptors. The position of the island of Calleja Magna in the rat brain makes it possible to inject dopamine D₃ ligands into the vicinity of these D₃ receptors, and to study their behavioral role, with no concomitant action on D₂ receptors. We studied the effects on body temperature and on locomotion of unilateral microinjections of D₂/D₃ receptors ligands into the island of Calleja Magna and into the adjacent nucleus accumbens. The results show that D₃ agonists injected into the island of Calleja Magna decrease body temperature and that this effect is potentiated by simultaneous injection of the D₁ agonist SKF 38393. D₃ agonists have no effect on locomotor activity in the island of Calleja Magna. In the nucleus accumbens, the D₃ agonists have only weak effects on body temperature, but, when associated with a D₁ agonist, strongly stimulate locomotor activity. The effects on body temperature of unilateral microinjections of dopamine agonists into unilaterally dopamine-depleted animals are the same as those in nondepleted ones. This indicates that the D₃ receptors are localized postsynaptically in the island of Calleja Magna.